Edit: I found it it occurred somehow when remapping modifiers with my keyd configuration. I remapped it to CTRL ALT ] and it works now.

It says on https://support.mozilla.org/en-US/kb/about-picture-picture-firefox#w_keyboard-shortcuts that the hotkey to toggle PiP is CTRL Shift ]. When I play a video it shows the PiP button, but the hotkey doesn’t work. The other PiP keys like seeking and volume work. I’m on Fedora 41 with KDE 6.3.3 & Wayland.