cpu usage spikes (~3 sec) to 70~90% is normal for most website with little flash content.
CPU usage stays high are still normal if you kept surfing website with huge amount of flash content
CPU usage staying high is NOT normal with website has little flash content.
Which case best describe your situation?
423 Real Memory is pretty high, UNLESS you have been using it for very long time and with significant amount of flash content.
If you have 4G memory, that might be different, app tends to use more when you have more physical memory installed.
In any situation, if you don't have too many extensions (As my personal experience, adblock, firegestures are compatible), firefox 3 beta 3 should be perfect option for you now.