ughaibu: Is it just not brainking you are having problems with ?

If it is everything then it is a problem lol. Is your computer old ?

Is there any chance of doing a System Restore ?

If you have re-installed Firefox AND still having problems logging here, or getting on the web i doubt it is Firefox. East thing to do as Rose say is try another Browser (preferably no IE lol). Chrome is a good choice.

If it is still problems with the web, it is either your connection or the pc. Connection you would have to talk to your provider providing you are sure your end is set up correctly.

PC problems could be endless, if it was ok, and now it is not, then it is case of finding out what. Which is a process of Elimination. IF you have no System Restore point all you can try is the things you should run regually anyway.
First thing to do is make sure all your virus protection is up 2 date, and run that. Run which ever system cleaner you use. Run Scan Disk. Run a Registry cleaner (Eusing do a good FREE one). Lastly Defrag it.

If you had some sort of error message it would be useful

MadMonkey: I am afraid there is something strange with Firefox 3.5 because it happens to me too. No matter which web sites I visit, after a couple of hours the browser starts to consume 50% (or more) CPU even if it is idle. A Firefox restart usually helps, until it starts again.
It never happened with previous versions of Firefox.

Fencer: It happens to me with an older version of firefox as well. Although it usually takes a few days to get there (faster if there's flash involved). It typically means it's consuming (and using) lots of memory which will not get swapped out.

Fencer: Mine also is the same.It get's very slow around 4 to 6 PM our time here.But when it is working it is very fast on this site.Could be due to my wireless connection