It seems weird that mozilla doesn't have something like phabricator so I can view a pretty diff of this easily.
Bugzilla (1998) predates Phabricator (2010) by about 12 years. It was written in Perl, which was the popular language at the time (similar to PHP around when Facebook happened, or JavaScript now). The first versions of Firefox (2002) predate Phabricator by about 8 years.
Fwiw I was using Bugzilla in 1996. Perhaps it was open sourced in 1998?
I feel it's functional but not great.
Upside: it is extremely flexible in ticket workflow design.
(Source: maintaining a 300-user JIRA/Confluence instance at work)
Phab, IIRC, kind of forces you to upload a patch and not some random file. Could be remembering wrong, though.