Firefox has started choosing the wrong language (randomly so far as I can tell) for spell checking.

A google search found many references to the problem, with suggestions to remove the unwanted dictionaries from firefox's addons.

But there were NO dictionaries in firefox's addons, not American English and not any other. But firefox was still finding foreign language dictionaries to use for spell check.

So I installed the firefox addon for an American English dictionary. But that made no difference. It still selects either French or Canadian English most of the time.

I expect my Centos system has foreign language dictionaries installed in some other package and firefox manages to find and use those. I don't need foreign language dictionaries on this computer, so if I knew what/where they were, I would probably uninstall them.

The real question is how to make firefox consistently spell check using American English. If the best way to do that is find and uninstall the other dictionaries then how do I do that? If there is some other way to force American English, what is that?

For a one time force of American English, I followed the instructions to right click in the box where the spell check is occurring and select language then select the desired language. Usually my selection doesn't take and that gets me to (or back to) Canadian English. If I try that several times, it then takes and I have American English for that one dialog, then back to French or whatever for the next.

Go go into a text box such as the one you use to answer this post. Right-click on the text box and a menu will appear. You should see an item called Check Spelling and a check mark should appear to the left of this item. If the check mark is not there then click on the item and it should appear. When it appears you should see another item called Languages appear below the Check Spelling item. Click on that one and then you can select the language you want to spell check. If you don't see the language you want to spell check then click on Add Dictionaries... and add the one you want. You can remove unwanted languages by removing the relevant files in the .mozilla folder.
jdk

Thankyou, but ...

Yes, that is what I described in the last paragraph of my post. The thing that takes a few tries and then lets me do a one time override of the language choice. Then it reverts to apparently random.

Can you be more specific?

I searched the whole directory tree starting from ~/.mozilla and I don't see anything that looks language specific other than (deap in a sub sub directory) the
en-US@dictionaries.addons.mozilla.org
that (in my first post) I described adding.

Like the solutions that google found, you are telling me to remove unwanted languages. But where are these unwanted languages hidden? The spell checker obviously can find them. But so far I can't.

It's hard to tell you because I don't know much about your system. All I can say is that the language folders are here on my system:
Obviously the folder under /firefox will be different on your system. Try looking for the extensions folder in .mozilla.
jdk

Other than the name of the profile directory, that is all the same on my system.

That extensions directory is where the en-US dictionary went when I tried (as described earlier) installing that to guide firefox's choice.

Nothing else was in that extensions directory.

Wherever firefox is finding foreign language dictionaries, it is NOT there.

On my system (Kubuntu 12.04) there's an "/usr/lib/firefox/dictionaries" folder. It has dictionaries for the different languages you describe.

The dictionaries don't appear to be owned by a package though the folder itself appears to be a part of the firefox package on my system...

I would say it's probably safe to manually remove them if you have a similar configuration as myself. I found the dictionaries with the mlocate package command locate.
EDIT:

After removing the dicts I had to restart Firefox for it to take effect. At this point I think I'm going to remove all of them and install the dictionary through addons.

1 members found this post helpful.

Thanks for the help. I was so busy I didn't have time to try it for a while.

On my system /usr/lib64/firefox/dictionaries was a symlink to /usr/share/myspell which contained US, GB and CS versions of en_ dictionaries together with a lot of other country's en_ dictionaries that were each symlinks (almost all to the GB version). I deleted all that except en_US.

But that did not fix the problem. It still randomly switches to Canadian English but no longer has that dictionary, so every word is marked as mis spelled.