I had talked about computer starting to lag after sometime when using Firefox over a long time. The reason is that Firefox has a problem of memory leaks. Read here for more. And I have also posted a couple of solutions for those facing this problem. Other than the link given above, you can check easy solutions here and here. This was a problem for those on Firefox versions 3.5 and earlier. Yes, you heard it right! That was a problem and it has been cleared off in their current Firefox 3.6.
The memory leak problem was so irritating that I had seriously considered shifting to Opera or Chrome but had to remain on Firefox because of some addons that I was using wasn't available for the others. But there still was a nagging thought in my head about this complaint about Firefox. It can really become irritating when your simple click action takes 10 seconds to execute instead of one.
Later I upgraded to Firefox 3.6 and to my surprise the problem is all gone. No more lagging of computer even if I have opened more than 20-25 tabs and left Firefox open for hours. It's a great relief to me and hundreds of other Firefox users who have been facing this problem. Now we have one more reason not to leave Firefox!
For those who haven't updated to the latest version can do so from here