I recently noticed that clicking on links in Thunderbird fails to open a browser window. It worked a few days ago, so I’m guessing a recent update must have had something to do with it.
There are bug reports, and there are quite a few workarounds, some of which are complex and none of which are elegant.
Nor are they necessary. The problem is caused by Thunderbird’s network protocol handlers. In 13.0.1, the warn-external entries are all set to false for some reason. Go into Preferences, then click on the Advanced tab. Click on the “Config Editor” button and confirm the dire warning.
Search for the phrase “handler.warn,” and change the values referencing http, https, and ftp to “true” by double-clicking.
Close the windows. Next time you click on a link, Thunderbird will ask you to choose a browser, and you’ll be set.
I’ve no idea why it’s going out the door that way, but at least it’s an easy fix.