www/firefox: update to 90.0.1 (rc1)

Release Notes (soon):

https://www.mozilla.org/en-US/firefox/90.0.1/releasenotes/

While here, try to fix menu display in dark mode[1], see

https://bugzilla.mozilla.org/show_bug.cgi?id=1720441

PR: 257225 [1]
Reported by: Graham Perrin [1]