firefox: Update version from 90.0 to 91.0

From the [release notes][1]:

> HTTPS-First Policy: Firefox Private Browsing windows now attempt to
> make all connections to websites secure, and fall back to insecure
> connections only when websites do not support it.

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

firefox.be0

@@ -2,7 +2,7 @@
 
 #export BEE_TMP_TMPDIR=/dev/shm BEE_TMP_BUILDROOT=/dev/shm/bee-root BEE_MAKEFLAGS='-j80'
 
-# BEE_VERSION firefox-90.0-0
+# BEE_VERSION firefox-91.0-0
 
 #https://ftp.mozilla.org/pub/firefox/releases/$pkgversion/SOURCE to find the right download link
 SRCURL[0]="https://ftp.mozilla.org/pub/firefox/releases/${PKGVERSION}/source/firefox-${PKGVERSION}.source.tar.xz"