This is likely caused by the same reason as bug #196 - Thunderbird doesn't pick up necessary file associations. Probably we should just rebuild it with "--enable-gio".

As far as i can see it is already built with it
https://abf.rosalinux.ru/import/mozilla-thunderbird/blob/rosa2012lts/mozilla-thunderbird.spec

Indeed, Thunderbird already uses gio.

It seems that when opening help page, Thunderbird looks for "x-scheme-handler/http" association which is not set anywhere in out mime databases.

For me it helps to add the following lines to the /use/share/applications/mimeapps.list file (owned by shared-mime-info package):

x-scheme-handler/http=firefox.desktop;
x-scheme-handler/https=firefox.desktop;

*** Bug 196 has been marked as a duplicate of this bug. ***

Probably the most correct way to fix this would be to add "x-scheme-handler/http" and "x-scheme-handler/https" to firefox.desktop file. After firefox installation/update these associations will appear in the mime database.

*** Bug 473 has been marked as a duplicate of this bug. ***

Advisory: 
'Help' in Thunderbird didn't launch web browser since no browser was associated with "x-scheme-handler/http" and "x-scheme-handler/https" mime types.

This fix associates Firefox with these types.

Build lists:
https://abf.rosalinux.ru/build_lists/743967
https://abf.rosalinux.ru/build_lists/743968

firefox-10.0.7-2
************** Advisory **************
'Help' in Thunderbird didn't launch web browser since no browser was associated with "x-scheme-handler/http" and "x-scheme-handler/https" mime types.

This fix associates Firefox with these types.

**************************************
QA Verified