Because it's almost obvious that either Thunderbird or Firefox freeze my whole system after some time, I'd like to not use FF at the moment and let Thunderbird run only for a short time as long as I check e-mails. Thunderbird web links should be opened with opera from now. For that I went in Thunderbird to Preferences->Advanced->General->Config Editor and set the following:
network.protocol-handler.app.http -> /usr/bin/opera network.protocol-handler.app.https -> /usr/bin/opera network.protocol-handler.warn-external.ftp -> true network.protocol-handler.warn-external.http -> true network.protocol-handler.warn-external.https -> true
Moreover I changed in Gnome in System->Preferences->Preferred Applications Opera as web browser and I checked in FF that it should warn if FF is not the default browser (obviously it isn't anymore).
Unfortunately all of these settings doesn't work. Thunderbird still opens web links with FF.
Any explanations why? (I already restarted Thunderbird although I don't have to, but no effect)