www/firefox: update to 95.0 (rc1)

Release Notes (soon):
  https://www.mozilla.org/en-US/firefox/95.0/releasenotes/

This firefox release includes a not-yet-released snapshot of dav1d,
which is not fully API-compatible with the released version. Switch
to the bundled dav1d/aom until dav1d released have caught up.