Welcome to your personal internet.

Meet Natsumi Browser (or Natsumi, for short) - a browser mod that truly puts you in control.

Natsumi Browser (or Natsumi, for short) is a skin made for Firefox and its forks that adds lots of polish to the design and adds new features to further enhance the browsing experience, while retaining customization options to allow you to fully make things yours.

Natsumi is NOT a standalone browser. It only supports Firefox-based browsers that are not known to be hostile towards user personalization.

Natsumi has lots of features to enhance your browser - so many that I can't list them all here!

Natsumi keeps your browser's design familiar while adding a modern touch to it. Not only that, it's also extremely customizable, so you can make it truly yours. No "best practices" or authoritarian design ideologies to stop you, just a browser that works with you and not against you.

Almost all customization options are accessible through a dedicated preferences pane, so you don't have to mess around with advanced preferences.

Natsumi applies a fresh new coat of paint to your browser's URL bar, making the current URL stand out more and changing colors dynamically based on context.

Natsumi's Picture-in-picture adds a more modern look to Firefox's PiP to make it more consistent with the rest of the browser. You can even scroll to reposition the PiP window, too!

Natsumi lets you control your media through the Miniplayer located on the sidebar. And with support for multiple websites at a time, you don't have to make your tabs fight for a spot.

Give Firefox's PDF viewer a fresh coat of paint! Natsumi includes tweaks for the default PDF viewer, giving it the modern look it really deserves. And with compact view, you can focus on the file at hand when you don't need the extra tools.

Not a fan of multiple toolbars? Craving more horizontal space? Natsumi adds it to your browser of choice.

And of course, you can always use the Multiple Toolbars layout. Natsumi doesn't force things on you.

Take full control of your browser's keyboard shortcuts! Natsumi lets you customize your shortcuts to your likings and even lets you choose how it should handle shortcut conflicts between the browser and websites. No more being forced to show mercy to websites when shortcuts conflict.

Does your browser have Workspaces? Or another toolbar that can be used in the sidebar? Natsumi adds extra features to your browser for a more complete experience.

Some Natsumi features require JavaScript to work. You can still use Natsumi without JS support at the cost of losing some features.

You will need to install Natsumi by copying its files to your profile's chrome folder.

Every release of Natsumi from v5 comes with an installer binary. You can run it to install Natsumi Browser, Natsumi Browser Pages and Natsumi Append to your browser and profile of choice.

Download and run windows-installer.bat as Admin and follow the on-screen prompts.

You can get this file from the Installers folder in this repo.

You can install the CSS-only version of Natsumi through Sine.

Remember to take backups of your userChrome.css and userContent.css files before proceeding!

1. Copy natsumi-config.css, userChrome.css, userContent.css and natsumi folder to your chrome folder.
2. That's it - Natsumi (CSS-only) is installed!

Installing Natsumi Append allows you to use the full feature set of Natsumi Browser, but it's not required for the core parts of Natsumi to work.

Natsumi Browser needs to be installed before you can install Natsumi Append.

1. Install fx-autoconfig to your browser. You need to install the contents of both program and profile/chrome, follow the instructions in the fx-autoconfig repository.

   Note: The guide doesn't mention the module_loader.mjs file, but you'll need this for fx-autoconfig to work. Otherwise, Natsumi's modules will show in your scripts but won't actually be loaded.

   Flatpak users: You may need to follow steps 1 and 2 on here, but replace app.zen_browser.zen with the package identifier of your browser (e.g. one.ablaze.floorp).

2. Copy the following to chrome/utils/chrome.manifest:

   content userchromejs ./
   content userscripts ../natsumi/scripts/
   skin userstyles classic/1.0 ../CSS/
   content userchrome ../resources/
   content natsumi ../natsumi/
   content natsumi-icons ../natsumi/icons/
   

3. Create and enable userChromeJS.persistent_domcontent_callback before restarting (don't worry, you can do this later. This config is needed for the preferences page to work.)

4. Go to about:support and clear your startup cache and restart your browser (this may not be required but it is still recommended).

5. That's it - Natsumi Append is installed!

After following either of the above steps, if you are interested in ensuring that you upgrade to the latest version of Natsumi as new changes are released, you can simply clone the repository into the chrome folder. This can be followed up with git pull from time-to-time.

These are the configs you can use to tweak Natsumi Browser. If you want to tweak the animation duration and delay, change the variables in the config.css file.

If you have Natsumi Append installed, most of these can be tweaked through the preferences page.

All browsers must be based on Firefox 137 or above.

Sure! Just paste it right below the Natsumi Browser loader (userChrome.css) and you can use your own userchrome alongside Natsumi Browser.

You can disable some features using the configuration options.

If that doesn't help, you can remove certain modules (CSS files) from the natsumi.css file. However, you will need to keep certain files (preload, postload, natsumi-config) for Natsumi to work properly.

As of Natsumi v4, Natsumi has dropped all support for Zen Browser. I have no plans to continue support whatsoever due to the browser's main developer's stance on personalization, as well as various other reasons.

Additionally, it'd be a hassle to make things compatible with Zen, mainly because it is just so heavily modified from the original Firefox that I may end up building a whole new skin just for compatibility unintentionally.

Please do not try to port Natsumi to Zen. I have my reasons why I left it, and I would really appreciate it if you respected my decision.

Natsumi does not support Midori and will not do so for the foreseeable future due to ethical concerns. Please note that this is in no way related to Zen Browser being unsupported.

For more information, refer to this blog article.

There is no fixed schedule for updates, it all happens randomly.

No. Natsumi is made on top of Firefox, so it only supports Firefox and (most of) its forks.

If you try to apply Natsumi on Chrome/Chromium-based browsers like Arc, you can't apply userchrome CSS files, making Natsumi completely incompatible with those browsers.

Although Natsumi is designed to be as customizable as possible, there may be some opinionated design choices that I may not change. In the end, Natsumi is still my personal browser skin.

If you would like me to add options for certain aspects of the browser, please open a feature request and I will consider it.

If your browser is open-source and licensed under the GPLv3 or later (or AGPLv3 or later), then yes. Otherwise, please contact me so I can grant you permission.

Natsumi Browser uses a system where the userChrome.css file acts as a loader that loads the skin, instead of being the file that contains all rules. This way, it's easier for users to quickly enable and disable custom CSS.

meow :3

Make sure both layout.css.backdrop-filter.enabled and layout.css.backdrop-filter.force-enabled are set to true. Also make sure that you're using WebRender and hardware rendering.

These are custom options which you need to create. Type in the exact name, then press the plus button on the right to create the config.

You can also access most of the configs through the browser preferences page if Natsumi Append is installed.

Please check that you've installed fx-autoconfig and userChromeJS.persistent_domcontent_callback is true.

This may be because you do not have the Proton design (i.e. the default Firefox design) enabled. Natsumi only supports Proton and does not support other designs such as Lepton or Fluerial.

You can fix this by going to Floorp Hub => Tab & Appearance in Floorp or Look & Feel in Waterfox.

This is not something I am able to help with. Natsumi is not tested with other themes in mind, so it is possible that installing incompatible themes alongside Natsumi may break one or more of them.

If you see this warning, it means Natsumi intentionally isn't supported on your browser due to serious concerns or severe compatibility issues.

When a browser's developers have stances that go against Natsumi's core values (especially regarding user customization) or a browser is heavily modified from Firefox to the point that it basically doesn't use Firefox's selectors, this can sometimes lead to me having to push out patches often just to address feature restrictions, compatibility issues and so on. This is not sustainable for me as Natsumi's developer, so I intentionally do not support such browsers out of caution.

Please consider using another compatible browser or uninstall Natsumi.

There may be userchromes that you're using alongside Natsumi Browser that breaks things. Please disable these then try again.

If the issue still persists, open an issue or report the bug to the developer through Discord.

Thank you to:

• asev for ZenCss, which served as the base for Natsumi Browser's base CSS for v1
• vicky5124 for the Tab Groups CSS used until v2.4.0
• AlexCookieDev for inspiring some of the animations I used in Natsumi v2 and v3
• InterstellarOne for the Liquid Glass style Natsumi icons

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

All screenshots displayed in this README are accurate as of December 2, 2025. If a feature was added, changed or removed after this date, it may not be accurately reflected in the screenshots.

Natsumi Browser uses code or files from other open-source projects. The original sources and their licenses are listed below.

• MX-Linux/mx-sound-theme-borealis

  License: GPLv3

  File used: Borealis/stereo/desktop-login.ogg, Borealis/stereo/K3b_success.ogg

  Changes made: none

• MrOtherGuy/fx-autoconfig

  License: MPL-2.0

  Files used: program (folder), profile/chrome/CSS (folder), profile/chrome/utils (folder), profile/chrome/resources (folder)

  Changes made: Modified profile/chrome/utils/chrome.manifest (changes shown using diff formatting):

  • ...
    - content userscripts ../JS/
    + content userscripts ../natsumi/scripts/
    ...
    content userchrome ../resources/
    + content natsumi ../natsumi/
    + content natsumi-icons ../natsumi/icons/