I'm pretty sure this is because Firefox is not using official branding (which results in the build ID getting set to all 0s). Mozilla bug 244485 is related.
-- Gecko/00000000 in Firefox's user agent string https://launchpad.net/bugs/68459 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
