Hello,
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->Prefered Applications Opera as webbrowser 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 explantions why? (I already restarted Thunderbird although I don't have to, but no effect)