TB add-on screwed up FF?

Thunderbird 78.12.0 (64-bit) and Firefox 78.12.0esr (64-bit) on Mac.

Background: I’m Swedish but always using English as system and software language. But since TB spellcheck also is English, it obviously pops up when I write mails in Swedish. At last, this morning, I decided to add a Swedish language add-on to TB in order to actually have a proper spellcheck. Downloaded and installed. No problems.

A minute later, I had to login to the admin of a web shop (using Firefox) and was greeted with this (oc.jpg). Thought that the site had been hacked, but tested in Chrome. There, the admin showed up as it should.

Updated and reinstalled FF to no avail. Searched for that Swedish language add-on in TB to delete that, but it can’t be found. Eventually got an account here at Discourse. First attempt to login presented this (moz.jpg) so I had to re-apply using Chrome (which I’m using now).

I’m absolutely baffled and have no idea how to fix this. That the installed TB add-on caused it is just a wild guess since everything seemed OK before that.

Can someone please give a suggestion?


If you installed a Thunderbird extension into Firefox, I would think it shouldn’t install, but if it is installed in Thunderbird, it shouldn’t affect Firefox.

For spelling in Thunderbird, you want to install a dictionary, not a language pack.

Those can be found at Language Tools.

  • To remove the Swedish language pack from Thunderbird, go to Preferences > General.

  • Select “Set Alternatives”

  • Select the language in the dialog that opens.

  • Select “Remove”.