Your system details do not tell, if the spell-checker is in-built or not. There's an add-on for Canadian English available (last updated dated 26.06.2013). Note the introductory text:

For some reason, Mozilla doesn't have a Canadian localization team, but I know that even the British dictionary doesn't quite cut it for the truly patriotic.

Are you perchance using it? In any case the discussion thread there would be worth having a look at.

Maybe asking there for guidance would be the best.

your fellow Canuck;)

Firefox has a built-in spell checker so I'm not sure how I could be using something else, but yes, I am using the built-in spell checker. Also, yes, it is the one and only Canadian English Dictionary "add on" I am using but as far as I know all the dictionary add-ons are strictly data files, not programs, which rely entirely on the built-in spell-checker for functionality and therefore the problem must lie (in all likelihood) within some change made to either the localization or spellchecker routines in the 22.0 build.

I don't see any discussion threads for the Canadian English dictionary except for two comments from the developer and several reviews, but I did read older threads elsewhere posting somewhat similar problems to mine for other languages for prior Firefox builds. Those threads have been closed with, unfortunately, no comments indicating solutions to their problems. Most say resetting Firefox to the default setting and reinstalling their dictionaries (the standard suggestion to "fix" problems which rarely if ever solves any posted issue in my experience) had no effect but they do say that the problem eventually "went away", suggesting that the problem there were experiencing was corrected in subsequent builds.

I meant to ask if you've noticed this problem yourself, and if not, if you're using the same build as I am.

Also, is there an external alternative to the Firefox spell-checker that you're aware of? I haven't found one, but something with grammar and context capabilities similar to that in Microsoft Word would be fabulous.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

If you do not keep changes after a restart or otherwise have problems with preferences, see:

• http://kb.mozillazine.org/Preferences_not_saved

Are you running Firefox in Private Browsing mode?

• https://support.mozilla.org/kb/Private+Browsing

Having the same problem, except it doesn't change language after I change tabs, it happens randomly. I have russian and english languages available in the menu, english is my default language, but for some reason it constantly changes to russian. Started in v20 I think.

What is the value of the spellchecker.dictionary pref on the about:config page?

Sounds that this pref isn't set correctly to the selected dictionary language, but stays at en-US causing Firefox to revert to that dictionary when the pref is read to determine which dictionary to use.

You can check for problems with preferences and try to rename or delete the prefs.js file and possible numbered prefs-##.js files and a possible user.js file to reset all prefs to the default value in case there is a problem with the file(s).

• http://kb.mozillazine.org/Preferences_not_saved

Similar problem here but I want spellchecking in en-US and it gets reset to UK or Canadian every time Firefox updates, or seemingly even more often than that. I have not been able to figure out exactly what it correlates to, I just constantly find myself annoyed to have the wrong spelling corrections, at least once per week it happens. And when it does, I right click and look at what it's set to and I see that it has changed itself again. When I use about:config to set it, it stays correct but then stops being correct the next time Firefox gets updated.

I am using Ubuntu 13.10 and always allow the latest updates. I don't think there is a problem with my prefs file because this is a brand new, clean installation of 13.10 (not an upgrade) and a brand new clean installation of Firefox that is included with Ubuntu. So if there IS a problem with the prefs file, then it's a problem with Ubuntu or Firefox standard install too.

Current Firefox version select an installed dictionary based upon a possible lang attribute set in the page (HTML) code.

This forum: <html class="no-js" lang="en-US" dir="ltr">

I don't think that's the problem. One of the most common locations where I see the wrong spellcheck language is when I'm using Facebook (but not only there). I don't think Facebook is setting a lang attribute other than en-US.

There is another problem that could be connected to this. Whenever I notice that the spellcheck language has changed again, I can right-click in a text entry box and choose "languages>" and then choose "English (United States)" and then the menu closes, and I notice that my choice is not actually set the first time. I always have to do that same thing a second time in order for the choice to stick.

Same problem, FF25, keeps reverting to US, when the config is set to AU: this is from the about:config page spellchecker.dictionary;en-AU

but it keeps reverting to US

Do you have more that one language installed in Firefox?

• Firefox/Tools > Options > Content > Languages

Thank you, cor-el.

I just found that I had both English (en) and English-US (en-US) installed. I did not manually install either of them... this is just the way the system is installed when Firefox comes already included in a clean install of Ubuntu 13.10. During the Ubuntu install when my language preference was asked I agreed with the already chosen option of en-US.

So I have now removed the plain "en" from the Firefox options menu that you directed me to, and I will wait to see if the spell checking problem continues.

It's still a bug, though... there is no justification for Firefox to spontaneously change my spellchecker default language under any circumstance.

I've been having the same problem (my language setting system wide is en_US, and everything checks out across about:config for it, but when I click in to the text box on Facebook, it switches spellchecker.dictionary to en_CA in about:config). I did notice, however, that if I reset the language after having it corrupted by clicking in to the text box, it seems that the fixed language sticks for the life of the FireFox instance (across multiple FB tabs). I also noticed that if I clicked in to the text box before FB had completed loading the page, my dictionary language never got overwritten, and that the un-overwritten setting appeared to stick across multiple FB tabs and page reloads. I'm no webdev or JS expert, but given that FB adds a bunch of event listeners and other infrastructure to the text boxes at different times during the page loading process, and given that most (including Google) other text boxes on the web don't present this behavior, it sure seems like there's some funky JS edgecase behavior FB is triggering that is either directly screwing up the settings, or causing Firefox to screw up the settings. I also noticed that FB has lang set to "en" (not "en_us") in their HTML, unlike pages that don't trigger this behavior.

Oh, I also removed all but en_US from the languages list in Edit > Preferences > Content > Languages and spent some time fooling around with the services.sync.prefs spellcheck related options as well as the social related options in about:config, but nothing I tried seemed to fix the problem.

Thanks for adding your post, silra. Come to think of it, it's possible that it's the combination of Facebook and Firefox in my case too. I can't be certain that I have never seen this problem behavior on other sites, but I will watch more closely to see if I can narrow it down to just FB.

I'll bet it has some connection to FB using 'en' instead of 'en-us' as you mentioned.

I have tried the same remedies that you mentioned but nothing so far has made any difference.

I sent a message to Facebook and they preferred to think that the problem was with Firefox because they don't see this issue on Chrome or other browsers. It was the same response I got over my other FB+FF issue (which is that all photos, when viewed in the single-photo view, are displaced about 30 pixels downward on the page relative to the rest of the layout. This has gone on for nearly two years and both FB and FF have pointed the finger at each other and not moved forward with it. I don't mean to complain too much, it's just an outstanding and rather severe problem that, oddly, no one seems to care about.