F9 doesn't seem to do anything for me on Linux... Neither on the posted page nor on HN.

What is it supposed to do?

There is no mention of F9 on this support page either:

https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perf...

Am I missing something?

yeah reader mode it is, didn't know it's different on Linux than on Windows and the support article listing it is here: https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perf...

I assume they are trying to enable Reader mode which is Ctrl+Alt+R

According to http://web.archive.org/web/20260317212538/https://support.mo... its

F9 on Windows

Ctrl + Alt + R on Linux

Command + Option + R on macOS

(It uses JS to only show the one for your platform but with view source you can see it mentions all three of these different OSes.)

So I guess the first guy is a Windows user and you other two use Linux.

> (It uses JS to only show the one for your platform but with view source you can see it mentions all three of these different OSes.)

There is a dropdown at the top-right to select the platform - no need to view source.

On mobile they’ve hidden that under “customize this article”, which I never would have even noticed if I hadn’t specifically known that there is some sort of dropdown somewhere, heh. But now we know :)

Probably. When available, reader mode can also be activated by clicking the little "page with text" icon on the right of the address bar.