Hi Hayden,

Did you delete en-us language from the Content panel in the Options? If so, it is the page display language and not spellcheck language.

Moreover, you can't remove default language dictionary of your browser. If you have English (US) version of Firefox, default dictionary is en-us.

So you should uninstall current Firefox and install English (British) version of Firefox (download here).

You can find the default dictionaries in the dictionaries folder in the Firefox application folder. Dictionaries that you install yourself are stored in the Firefox profile folder. You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

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

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

I have a similar problem. Firefox keeps resetting itself to the British English dictionary, but I want to use the US English dictionary. I have the default Firefox 39 for my US English Linux Mint 17 install, and spellchecker.dictionary is set to en-US, but Firefox still keeps resetting itself at random intervals to British English. I can download and install the en_US version of Firefox 39, but I'd rather just make this install work correctly. Is there a way to make my designated dictionary persist? Thanks!

A possibility is to set the spellchecker.dictionary via a user.js file to a predefined setting of your choice. Then at least you have the correct dictionary selected each time you start Firefox.

The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

You can create a plain text file named user.js in the profile folder.

• http://kb.mozillazine.org/Editing_configuration
• http://kb.mozillazine.org/user.js_file

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Adding a user.js didn't help. However, that prompted me to look in my profile directory and see if there were any other culprits. I opened the prefs.js file and found, on line 788, user_pref("spellchecker.dictionary", "en-GB"); . I changed this to en-US and restarted Firefox. It now seems to be correctly defaulting to US English; hopefully this will be persistent.

Hmm, that doesn't seem to have worked. Something, I don't know what, reset my dictionary to en-GB without me asking it to. Checking the prefs.js file again, I see it is set to en-GB. What would cause this file to spontaneously change? Is there some part of Firefox that is recalling an earlier (erroneous) setting and recurrently resetting my install to that? Most importantly, how do I prevent this from happening? I'd really like to set Firefox to one language per session, or ideally just one language ever, rather than having to do it at random intervals when Firefox feels like annoying me.

If you use a user.js file with that user_pref line then at least you should get this dictionary (re)selected on each start.

user_pref("spellchecker.dictionary", "en-US");

Restarting the browser to get the correct spelling dictionary is rather extreme. Does that mean that there's no permanent way to stop Firefox randomly selecting a different dictionary? I do a lot of work via typing in my browser, so this is a major problem.

There are extensions to change the dictionary that you can look for.

• Dictionary Switcher: https://addons.mozilla.org/firefox/addon/dictionary-switcher/

Doesn't that just accomplish the same thing as right-clicking and then selecting Languages > Whichever dictionary? I'm not talking about ease of of switching; I'm talking about getting Firefox to stop changing the dictionary by itself. Seems like that shouldn't require an extension to accomplish.

Yes, that is basically the same. I don't know what makes Firefox switch the dictionary.

Note that Firefox probably stores such a website related setting in the content-prefs.sqlite file in the profile folder as part of the Site Preferences. So if it keeps going wrong on specific website then consider to clear the Site Preferences or delete the content-prefs.sqlite (you keep the permissions deleting the file).

• http://kb.mozillazine.org/Profile_folder_-_Firefox

Not only is this problem not solved in Firefox, it now seems to be affecting Thunderbird. I'm now getting Thunderbird selecting UK English as its spelling dictionary all by itself. This appears to also be ignoring my user dictionary, meaning that composing messages in Thunderbird has just become significantly more difficult.

Someone please fix this problem!