I had this issue in the past:
Somehow I figured it out. Now I need to do it again on other computer and... the thread above is not enough. Mainly, when I am on the list of files (in Attachements->Incoming) there is no http or any alike content.
Last time I had to find firefox and changed it by pointing toward proper browser, for chromium it was /user/bin/chromium.
The problem is, this time firefox is not even on the list and yet thunderbird uses it to open links which freezes my old laptop. Firefox is just too heavy for it, chromium works just fine and i want thunderbird to use it.