Try typing `man:ls` for instance. I tried it on chrome as well, but this seems to be a firefox only feature.
Turns out that it's because KHelpCenter in some linux distros hooks to these uris. So typing this uri would autoopen these man entries in KHelpCenter.
x-man-page://man
version 125 isn't new enough for you?
Oh you mean firefox looks uses a protocol handler installed on your system to open an unusual link? Never heard of mailto: irc: magnet:? I think even steam had its own at one point.