Hijacked startpage in all my browsers

Hi all, need some help,

Since last week my web browsers have been hijacked and i can`t find a way to get rid of the spyware or whatever it is that`s causing it.
First, i have always used Google search as my startpage in all browsers but now whenever i close down the browser and re open it my start page changes to Yahoo search.

Also, i`ve noticed that everytime i open up my main browser (firefox) a strange buttom appears in the firefox toolbar. If i try to manually remove the button in toolbar settings, it always comes back when i re open the browser.

This is how the button looks and if i push it i`m directed to a web page from Ant.com as you can see.
Attachment 143785

Attachment 143793

Because the buttom somehow is related to Ant.com i suspected one of my firefox addons to be the cause, namely "ant video downloader" but i have tried to remove it and then re-install it without any luck.

I`ve also tried to uninstall Firefox and re-install but no luck there either..

I`ve searched my PC with Spybot Search & Destroy, Malwarebytes and my regular AV but the problem is still there..

Any suggestions?

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