Comment 30 for bug 136303

Today I thought I suffered from this bug because I had the very same symptom, but then I found out it was a completely different (and selfmade) problem. It turned out that for https links thunderbird was still trying to start a local copy of firefox that I just deleted. It wasn't configured as network.protocol-handler.app.https but as a mime type. To figure out if you have the same problem, just run this:

fgrep firefox .thunderbird/*default*/mimeTypes.rdf

and check the output for non standard pathes to firefox.