While in Firefox hit
CTRL +
SHIFT +
A and check if you have any unwanted addons in your browser. It looks like an advert addon.
If that does not help you, double check your
Programs and Features in
Control Panel for any unwanted software. Some adware will auto install these addons into your browser.
You should probably also scan you PC with:
https://www.malwarebytes.org/ for any potentially malicious software. A complete reformat is not necessary.
If Malwarebytes doesn't pick anything up, you should try scanning with Spybot Search & Destroy afterwards.
http://www.safer-networking.org/en/spybotsd/index.htmlYou can also review this guide, under
IV. Risks and Prevention:
Account Safety And D2jsp Information