www/firefox: update to 91.0 (rc1)

Release Notes (soon):

https://www.mozilla.org/en-US/firefox/91.0/releasenotes/

Removed patch-bug1628567 - problem sesems to have gone away (and patch
does not apply anymore)

(cherry picked from commit 2d661b18c2c37a2074b9dcb8df7a82c2c7ebc6b8)