ALWAYS use the latest version, Firefox is now up to 38, and if you are not fully updated from Microsoft updates, I would wipe the drive and reinstall xp, as you are most surely infected.
Want really bad lag? Try Windows (any version) NT 6.x and above.
Lag is the reason why everything post XP (so called modern OSes) turned to sh*te, and why I'll never buy another Windows license again, whether bundled with OEM hardware or NOT!
Btw, I'm surprised. Why FF3? If it's giving u problems, I highly recommend this:
It's like The BEST browser that was ever made (for XP). Too bad it's discontinued, but it did get patched for HEART BLEED, thankfully. And I don't recommend the newer Opera releases at all...