If you recently upgraded Thunderbird to v52 onwards and all links in emails stopped working (i.e. clicking on a link doesn’t open them in your browser) then you can fix it like this:
- In Thunderbird, go to Tools | Clear Recent History and clear everything (this just removes your usage history, not any emails), then
- Close Thunderbird, go into your profile folder (i.e. where the emails are stored) and delete all the places.sqlite files.
Tap-tap. Job done.
Source: https://bugzilla.mozilla.org/show_bug.cgi?id=1354007