LE:
The toolbar appears also if a doorhanger needs to be displayed: permission to access the camera/location for example.
Should be fixed by https://github.com/mozilla-mobile/fenix/pull/10306 in master
This is still happening (screenshot from yesterdays Fenix nightly)

I think it happened after I reopened a PWA I hadn't used for a couple of hours.
I noticed that opening an instagram story triggers this. Not sure if there are more ways to trigger it.
I'm not able to reproduce this anymore. Can QA check if this is still an issue?
@NotWoods I can easily reproduce on the instagram PWA, open a story tap through the end and after you exit the PWA will have a toolbar.
Yeah, it now has to do with videos because on fullscreen, we are hiding the toolbar, but then when fullscreen ends, showing the toolbar again and don't treat PWAs differently right now. You can get it to show again by clicking on a link within the PWA.
issue #10452 exists to deal with the full screen problem. Closing as a dupe of #10071 which was fixed by the pr Emily mentioned.
Most helpful comment
This is still happening (screenshot from yesterdays Fenix nightly)

I think it happened after I reopened a PWA I hadn't used for a couple of hours.