Firefox's memory leakage problems are well documented. What tends to happen
is the longer you browse using Firefox it uses up more and more memory until
your PC virtually strips.

This tip from gHacks should reduce Firefox's memory usage to fewer than
10MB. It's very simple to implement:

1. Write 'about:config' in your firefox address bar and hit enter
2. Right click and select 'new Boolean'
3. Paste config.trim_on_minimize in the field and hit enter again
4. Select True from the list
5. Restart Firefox

Once you've done this you should notice a speed improvement on your PC when
using Firefox