https://support.mozilla.org/en-US/questions/1424372 is one of the Thunderbird reports Anje and I have been struggling with in recent weeks. Chatting with Geoff this evening, layout.css.devPixelsPerPx can definitely be a cause, as Anje attempts to get at in that SUMO posting. https://support.mozilla.org/en-US/questions/1044747 describes the issue for Firefox.

A non-default value can block all the 115 based UI size controls. And Help > Troubleshoot Mode doesn't block the non-default value.

[edit]
If you are unable view Thunderbird, edit prefs.js in your Thunderbird profile and change the value for layout.css.devPixelsPerPx to -1.0

Otherwise, see instructions at https://support.mozilla.org/en-US/kb/thunderbird-115-supernova-faq#w_why-are-the-fonts-still-too-large-or-too-small-after-i-changed-the-density-and-font-size

[end edit]

Anje's instructions from SUMO ...

Check the Config Editor for this settings:

• Settings > General
• scroll to bottom and click on 'Config Editor' button

It opens in a new tab

• In top search type:''' pixels'''
• look for this line :''' layout.css.devPixelsPerPx'''
• default is a '''negative '''number '''-1.0'''

It should not be in bold text, if bold text then it has been altered.

• If you have anything else in that setting use the arrow icon on far right - click on it and the default is reset.

layout.css.devPixelsPerPx is preference that comes from Firefox and we don't use nor support.
Probably these users edited it in the past when Thunderbird UI wasn't really responsive nor scalable via built-in density or font size.
I don't think we have any control over that pref, and we can't even ignore it.

To rephrase comment 3...

layout.css.devPixelsPerPx is supported by Firefox, but is deprecated (no longer supported) for Thunderbird starting at version 115 in 2023.

Anything other than the default -1.0 may cause problems or inconsistent results now or in the future. Please use font-size and density scaling in Thunderbird.

Reference: https://support.mozilla.org/en-US/kb/thunderbird-115-supernova-faq#w_why-are-the-fonts-still-too-large-or-too-small-after-i-changed-the-density-and-font-size