phpBB3 is in not a single way americanised (or americanized, as the americans call it
). If you find american spelling, do not hesitate to report it to the phpBB 3.0 bug tracker
. Your problem seems to - as CoC said - relate to some kind of spelling checker you have installed. I am guessing it is the one included in firefox. So it's not actually related to phpBB at all.