Thunderbird on my Arch Linux installion was opening links in Chromium, even though everything else on the system pointed towards Firefox being the default browser.

Some people on the internet said to go into Preferences > Preferences > Advanced > Config Editor (yes, that’s preferences twice) and add the keys

• network.protocol-handler.app.http
• network.protocol-handler.app.https

and set their values to /usr/bin/firefox. This, however, did not work.

What did work was to set the keys

• network.protocol-handler.warn-external.http
• network.protocol-handler.warn-external.https

to true. This makes Thunderbird prompt you what program to open every time you click a link. Here you can choose Firefox, and also check “remember my choice…”.

Reference: https://www.ghacks.net/2013/01/20/fix-for-thunderbird-not-opening-links/