www/firefox: update to 76.0

• Add a workaround for Clang [1]

Changes: https://www.mozilla.org/firefox/76.0/releasenotes/
PR: 245422
Submitted by: tobik [1]
Security: aec18162-eaf3-4e73-a38e-3fd98bc47869
Differential Revision: https://reviews.freebsd.org/D24326