I've always had the issue with Xfce and Firefox that the screen would suspend while watching a video.
Note that when using Chromium the screen doesn't suspend as expected.
My first assumption was that it is because Firefox is not calling the correct DBus API in Xfce as described in #42 (closed)
I made a code change in Firefox to fix this which landed in Firefox 95: https://hg.mozilla.org/integration/autoland/rev/1a71a4f15928
Now I correctly get from the Power manager applet "Firefox is inhibiting power management" (or something like that as my desktop uses the French locale), BUT the screen still suspends!
After some investigation, I believe that this is because xfce4-power-manager doesn't inhibit DPMS when getting the inhibit request, whereas it does it when entering presentation mode. I made a small change to inhibit DPMS in this case (this pull request), and this fixes the issue for me, the screen doesn't suspend any more while watching a video with Firefox.
Does it look like a good approach to fix this issue?