If you're using KDE and upgraded to Thunderbird 3 lately you might have the problem that links in emails don't open in a browser anymore. For my case that happened even without a user visible error message.
If that sounds familiar for you, check the error console (Tools/Error-Console). If there is an error like:
Error: uncaught exception: [Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIExternalProtocolService.loadUrl]" nsresult: "0x80004005 (NS_ERROR_FAILURE)" location: "JS frame :: chrome://communicator/content/contentAreaClick.js :: openLinkExternally :: line 188" data: no]
Then Thunderbird just (silently) fails to launch the browser. Guido
pointed me to gconftool.
gconftool -R / | grep url-handlers told me
that some components of gnome still had Firefox configured as
http-handler (which is strange since it was rebranded to iceweasel ages
ago). Resetting them solved the problem for me.
The sad part of the story: x-www-browser, sensible-browser and KDE's http handler where all correctly configured and pointed to iceweasel. All applications behaved correctly only Icedove used a (for me) hidden setting pointing to firefox.