I’ve been using qubes for years now. I used to run tor browser of qubes (I know bad). Now that doesn’t seem to work anymore, and when I just try to use firefox onions don’t work despite all the about:config settings I’ve tired so far by googling.
Looks like your sys-whonix lost connection.
Have you tried to shutdown anon-whonix and then sys-whonix and restart them again to immediately check, if the addresses are working again?
After a fresh install, I’ve had problems with onions connecting because of the clock / time settings. Did you check that? Once I fixed the time, whonix established a connection.
Are you trying to use Tor Browser behind sys-whonix?
Indeed that will not work - the Tor Project have (at last) disabled Tor over Tor.
Otherwise, please describe your Qubes network configuration.
TBB works fine in the normal configuration in my experience.