This issue is so annoying, that I need to ask Firefox community what I can do. So, the problem is, that if I set custom home page called Infinity New Tab (Pro), then I will restart Firefox, then it reverts back to Firefox Home. I think, that this is completely unrelated to malware, because I'm scanning my computer very often. Are my preferences/settings corrupted? However the Infinity New Tab (Pro) is no longer available in addons.mozilla.org, so if you need .xpi file, then tell me, and I will attach that file here.