[PROMPT] not working in Firefox -Arch Linux

[cg00001]

[PROMPT] works in Chrome / Chromium but not in Firefox -my main browser.
I'm using it to play facebook videos with mpv.
Now that facebook changed the naming, only "Copy link" then using [PROMPT] works.
The only thing I get is shown in the pic.

[cg00001]

Thank you, works fine.