this is a big problem in win xp try downloading a new windows

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

The solution is easy. Install "Malwarebytes' Anti-Malware" free edition and use it. This program found malware in your pc. After that clean this malware. Finally instal Firefox again and enyoy it because this installation is correct and clean. Regards.