Since this morning on my brother pc when he tries to connect to any sites(such as amazon or youtube) using firefox (with chrome or edge ti works fine)it shows up a routerOs login screen. We don't know what it is and we never heard of it before (found this site searching about it). How can we remove it, and how did it install? yesterday the computer worked fine.
I've tried reinstalling firefox but it still shows up (not on facebook, it works).

Hi ,
I don't think the problem is your routeros , because you are fine with other browsers , try to clear cache and history of firefox , sometimes it would be happen on firefox specially with https sites