Add "plugin" and "plugins" to dictionary. Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1a2pre) Gecko/2008080103 Minefield/3.1a2pre

I disagree. The correct spelling according to http://www.merriam-webster.com/dictionary/plugin and other dictionaries is "plug-in. "plugin" is jargon and not (yet) correct English. You're free to right-click on it and choose Add to dictionary if jargon is OK for you. So the bug here is really that the spell checker doesn't offer "plug-in" as a correction for "plugin". I don't know if hunspell can be coaxed into doing that; my en-US.dic in a recent Firefox 3.1 nightly doesn't have any hyphenated words in it and it doesn't suggest other hyphenations, like "cop-out" for "copout". I added "cop-out" and "plug-in" to my en-US.dic (make a backup first!) and spelling seemed to do the right thing, your mileage may vary.

So why the hell Firefox use hyphenated "Add-ons" at menu item "Tools > Add-ons" but non-hyphenated "Plugins" as tab name in the "Add-ons" window. The point is spell check in Firefox is mainly used on forums, chat, emails etc. There the common spelling used is "Plugin" and "Plugins". And spell checker should not be considered like merriam-webster or other dictionaries.

(Please relax, there's no need to swear.) Firefox has misspelled "plugin" since before it existed from the Netscape days, but two wrongs don't make a right. I note that most of my plug-ins in Tools > Add-ons > Plugins (including "Mozilla Default Plug-in") spell it correctly. In my opinion, the Mozilla spelling dictionary is most useful to the most users when it's a close match with the proper American English that you find in... dictionaries. en-US.dic does allows "email" as well as "e-mail", so the actual developers working on this code may find your argument compelling.

"plugin" is so frequently used in online English that it's on Google Chrome's word list, which was auto-generated from Google's database. Adding as part of Bug 479334.