Whenever Firefox played videos or - actually more relevant - I watched the shared screen of a colleague, after some time the screenlock would start. This is especially annoying, since I have to enter my password to unlock again.
In my status way waybar there is an idle inhibit switch which I could then use to prevent the lock from activating. That this switch worked led to the conclusion that there is something with Firefox that prevented this from working. Behind the scenes, there are two components that have mixed responsibility when it comes to locking / suspending / sleeping - responsibilities which overlap, and are not completely clear in the end.
