Check your list of home pages to see if a home page is causing the problem. You can try changing your home page to see if that helps - How to set the home page.

Another possible cause is an extension or theme causing the problem. Try the procedure in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article.

You can also check the Target filed in the Properties of the Firefox desktop shortcut to make sure that noting is appended after the firefox.exe program, except a possible closing quote.

You may also try to uninstall and then reinstall firefox from your computer. Doing so will usually solve many of these types of problems.

On the contrary, re-installing Firefox rarely solves problems that are caused by an incorrect preference setting, an addon, or Malware - which is probably the case with the OP.