You used "Reset Firefox" as described in this article: Refresh Firefox - reset add-ons and settings?

Have you tried reinstalling Firefox over itself from http://www.mozilla.org/firefox/fx/#desktop

Is there an error in the Board's detection of your OS -- it is reporting NT4, which is not a supported OS version for Firefox 3 or later. ??

Thanks for your response.Unfortunately when I try to download anything my browser closes so I can`t reinstall Firefox. Many of the fixes say "go to Tools & Options" but when I click on Options the browser again closes. I seem to have lost access to IE some while ago and have been using Firefox OK up until recently, but then my computer started to go very slow and the Task Manager keeps showing 100% usage of the CPU. When I start the pc with only the desktop displaying, the CPU usage is 60-70% and around 35 processes are running. Is this normal? When I open Firefox the usage increases rapidly until the CPU shows 100%. It was while resetting Firefox to try and sort this usage problem that I lost my download ability and the use of "tools" then "options" and also urls that are typed in.(bookmarks & history ok) Any advice welcome.

Have you ruled out a malware infection? You may want to try some supplemental security scanners. These two tools are highly regarded:

Malwarebytes Anti-malware : http://www.malwarebytes.org/products/malwarebytes_free

SUPERAntiSpyware : http://www.superantispyware.com/

If you cannot download those on your PC, download them to a USB flash drive on another PC and then use that to run them on your PC.

Thank you again.I ran the malware check & everthing was OK. I have now discovered that the process that is taking all my memory is "sump.exe" which only shows using 55K but when I end it my CPU usage drops very dramatically. This process is part of (amazingly) "Speed up my PC" from Uniblue.

However I still have the same browser problems and when I tried to reload this from a USB flash as you advised, I received the message "can`t create temp folder archive" and was unable to move on from there. I am at a loss as to what to do next...

Does it make any difference if you copy the installer from the USB drive to the desktop before running it?

Does Internet Explorer work normally?

It might be useful to get some "hands on" assistance.

I had got as far as finding that my default Firefox had empty files in "profile". However I took your advice and copied the USB installer to desktop before actually installing it. Fantastic...all my Firefox info has now been restored. It seems as though my old Firefox default had lost data whilst I was trying to sort out my CPU congestion problem. Thank you again for all your help.

I do agree with all of the ideas you have offered for your post. They are very convincing and can certainly work. Nonetheless, the posts are very brief for beginners. Could you please extend them a bit from subsequent time? Thank you for the post.