Firefox has the nasty habit that it will force-dereference symlinks when locally opening files (i. e., opening an annexed document will cause it to be opened in .git/annex/objects/…). Since this will break relative links within HTML files, this would make Firefox pretty useless when working with a git annex containing HTML files. (Apparently this behavior is desired upstream and might not be fixed.)
Seems I’m not the only one who would like to work with annexed HTML files, though. On the Debian bugtracker, another user shared a handy shim which can be used in LD_PRELOAD and which will force Firefox to open symlinks in-place. Thought I’d share this here in case it’s of use to anyone.