There are probably sites doing this maliciously but the cases I've seen are weird things like only some subreddits and even then not always. I suspect there's a

There are probably sites doing this maliciously but the cases I've seen are weird things like only some subreddits and even then not always. I suspect there's a bug somewhere in a commonly used web framework or in Firefox mobile itself. This site for example doesn't have the issue in the desktop version of the site, only the mobile one.