Just found this in the gentoo forums.
Firefox will cache pages which leads to memory usage of upto 130 or
200MB as and when you open new webpages. To overcome this problem,
1. Type `about:config` in firefox addressbar
2. Modify the variable `browser.cache.memory.capacity` (or create new one)
3. Change the value to X where X is cache size in KBytes.
For my system with 128M RAM, I prefer this value
(Copied from my blog)
Sridhar - http://www.cs.annauniv.edu/~rsridhar