On my old pc - Dual Core 2,600 Mhz, 2 Gb ram, 1024 video Geforce GTX-250, windows XP sp3, my firefox game was running at 10-14 FPS and 10-20 in Chrome (with people turned off, in some empty dungeon). Game was taking 70-99% CPU.

Then i brought new pc - Core i5 3000 Mhz, 4 Gb ram, Geforce GT-640 2 Gb video, Windows 7 - Firefox works all the time 30 FPS. Game taking 20-30% CPU in city center with turned on players, while moving around like crazy.

Morale - chage your PC for better perfomance.