Sometimes, in Linux, after an update or a brand new install, Thunderbird won't open http links (e.g., with Firefox). This might depend on some default configuration value of your distribution.
However, the quickest, and, as far as I know, distribution independent solution, is to manually set these preferences in the prefs.js file:
user_pref("network.protocol-handler.app.ftp", "firefox");where "firefox" can be replaced with your preferred browser.
Alternatively, you might set these settings with the config editor available from the "advanced" tab. You'll probably need to create new entries for these preferences (of type string).
Hope this helps, at least until the problem is fixed in your distribution :-)