Do you have an older version of Firefox installed already? If so, go to Help | Check For Updates and update it that way.

If you do have an older version installed, go to Tools | Add-ons | Extensions and see if you can see Ask Toolbar in there. If so, remove it.

If you don't have Firefox installed at all, try and download it from here: http://www.mozilla.com/

If it still doesn't work, download Malwarebytes from http://www.malwarebytes.org/products/malwarebytes_free and run a full system scan to check for malware.

Temporarily disable your anti-virus application for only as long as it takes you to download Firefox.

I'm not sure how to disable the application.