Firefox has a nasty problem with slower processors and XP's usage of RAM it seems. If your computer has a slow(-ish) processer and not much RAM (less then 512mb's when running XP) it seems to be obbsessivly slow. This is just from personal experiance as we all know how badly XP manages RAM and how well it manages clock cycles (or processor time).

So either get more RAM, a new computer or get one of those RAM cleary out things to wipe your RAM every so often, why doesn't it affect IE? Because IE has a special place in windows heart being intigrated and all so get priority treatment in the RAM and processor areas

Either that or of course Firefox copied your internet connection settings over incorrectly xD