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[Connor]

Run a scan just like the others have said but before you reinstall anything check the extensions tab and disable anything that has quite a few numbers/letters or just looks plain odd. Usually you'll find it there if you don't get rid of it using a virus/malware scanner. I'd recommend that you ether use Microsoft Security Essentials or Avast as your virus scanner along with Spybot Search and Destroy as your Malware/Pup/Other crap scanner.

Edit: Extensions on firefox can be found here using the screenshots as a guide

1) http://i.imgur.com/Bv030sN.png
2) http://i.imgur.com/EIfDw75.png

I would suggest that you just delete all extensions.

If this still doesn't resolve your issue, download an extension called "Add Blocker". This ~should~ block it if the above fail.

Edit2: For a better guide, look here

https://support.mozilla.org/en-US/questions/941136

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