DO NOT USE THIS ADDON.
I skimmed through the code, and what I found was appalling. It tries to steal your steam username and password by presenting a fake login page by redirecting you every time you go on one of the official steam domains. It also does some other dodgy stuff, like trying to extract saved passwords and active login cookies. Of course only americans would fall for this..
Honestly depressing that you thought you would get away with this.