Very very slow

Firefox has become very slow. I have made fresh install.
Also I have disabled session store, cache and anti-phishing. Atill firefox is freezing momentarily even while changing tab.

I have switched from Opera to firefox only because Opera9.51 was slow. But it is same here also.
Opera used to consume 30 to 40 MB memory but firefox is consuming more than 100MB of RAM.

I removed all the data form the Pendrive, formatted(hadn't formatted for 3-4months) and firefox seemed to speed up. (The firefox files were fragmented) Still it is slow.
Any hints to speed it up