Sounds like Malware.

Install, update, and run these programs in this order. They are listed in order of efficacy.
(Not all programs detect the same Malware, so you may need to run them all to solve your problem.)
These programs are all free for personal use, but some have limited functionality in the "free mode" - but those are features you really don't need to find and remove the problem that you have.
Note: If your Malware infection is bad enough and you are mis-directed to URL's other than what is posted, you may have to use a different PC to download these programs and use a USB stick to transfer them to the afflicted PC.

Malwarebytes' Anti-Malware - http://www.malwarebytes.org/mbam.php
SuperAntispyware - http://www.superantispyware.com/
AdAware - http://www.lavasoftusa.com/software/adaware/
Spybot Search & Destroy - http://www.safer-networking.org/en/index.html
Windows Defender: Home Page - http://windows.microsoft.com/en-US/wi.../windows-defender
Also, if you have a search engine re-direct problem, see this:
http://deletemalware.blogspot.com/2010/02/remove-google-redirect-virus.html

If these don't find it or can't clear it, post in one of these forums for specialized malware removal help:
http://www.spywarewarrior.com/index.php
http://www.spywareinfoforum.com/
http://bleepingcomputer.com

Hello,

Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• On Windows you can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac you can open Firefox 4.0+ in Safe Mode by holding the option key while starting Firefox.
• On Linux you can open Firefox 4.0+ in Safe Mode by quitting Firefox and then going to your Terminal and running: firefox -safe-mode (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Once you get the pop-up, just select "'Start in Safe Mode"

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help other users who have the same problem.

Thank you.

The small Google doodle image of a calculator with a face of a Woman is to honor Shakuntala Devi "the Human Computer". The full doodle is on google.com

Which you are aware of at https://support.mozilla.org/en-US/questions/976383