Does that happen in Firefox safe mode? If not, it is probably an extension causing the problem. Do you have Print Preview open at the same time? Apparently, that can cause problems for some people. Are you accessing a page that uses any plugins at the time? Some Windows users have claimed that holding down the Windows key for one minute can help eliminate the problem.

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) as a test.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/

Hi thanks for the advice. booting firefox in safe mode made no difference but i started the laptop in safe mode with networking and firefox worked fine. I dont have any plugins and print preview isnt on.Holding down the windows key didnt help. more advice would be appreciated

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

What security software (firewall, anti-virus) do you have?

Some can have some security protection and scramble the keyboard input.