Yes mozilla is a little heavier than opera... But it start up quite fast if you put it in memory"a sec or 2", thats also only reason why IE starts up fast btw... Cause its allready lying in memory.
Though its a little bit slow to start up, its damn fast.... And yeah its open source.............. You cannot port opera, since its closed :\ And not very likely Opera themself will do an Amiga version...
