Thunderbird links not working

All of a sudden, when I click on a link in Thunderbird, nothing happens, where it used to open a tab in Firefox. Left clicking and selecting “open in browser” also doesn’t work.

I checked in Firefox to make sure I had it set as my default browser, but when I click on an html file it opens in Konquerer. Is there another setting I should change? If so, what? Where?

Bug report with solution. Hope it works for you too.