What antibot "mod" (phpBB uses extensions these days) are you using, can you post a link to it please? Also, check your cookie settings, the Cookie domain shows
and it should be
, note the leading dot. See Fixing incorrect cookie settings
. I don't believe the issue you're having with FF is phpBB related but go ahead and post in the bug tracker if you wish.
IMHO I'd say you'll be better off using the built in Q&A with a non-searchable (as in Google) Q&A. That along with setting the NRU group post threshold to one so that each new users first post has to be approved will stop 99% of bots. Human spammers will be trapped until you approve their first post or you kill them.