What video card do you have? The only problem I found when I Googled was related to Nvidia cards.

Found it through a terminal session.

I suggest you ask in the Ubuntu forums. You might also try another distro on a live CD and see if you get the same problem.

It might just be something as simple as a FF setting. Are you sure you have not managed to turn off Javascript somewhere?

Do other browsers work OK? If Epiphany Gecko is still in the repos trying it might narrow down the possibilities because it uses the same rendering engine as FF (they may have switched completely to the newer webkit based version of Epiphany). I think it has its own copy (rather than using a shared library), but if it also does not work it is a clue.

All you have to do is activate the "console" part of Firebug and reload the page. Any Ajax requests will be shown there.

It may offer more clues, like if you see a request and an response but nothing loading on the page then at least you know that the requests are happening.

I am using the latest install of Ubuntu's FF and have had no issues.

That is the same version I have installed.

It could be that you have some other setting that is blocking requests.

Try installing Opera and see if Ajax requests work using it... if you get the same issue in Opera then you know it is something else and not the browser itself.

Do you have a firewall or anything like that running?

If you have not got Flash go here <url:https://help.ubuntu.com/community/RestrictedFormats/Flash>

Before a re-install I would have recommended looking for a Linux guru in your area to have a look at it.