this seems to be the case for me as well ( moto e4, android 7.1.1, firefox 68.7.0 ) and it has been since this was introduced over on boingboing.
my phone has also been hanging quite a lot lately, and it seems most prone to do so when the vibrate notification popup happens. it’s as if its somehow eating a lot of resources, or sitting in some tight loop.
it’s fine if not much else is going on on my phone, but if i have a bunch of tabs open or other apps – then it’s almost inevitable that i click a “like”, the popup starts to happen, and my phone hangs. ( has happened twice today; reboot required. )
i’ve looked for settings to disable it permanently ( in my user profile ) but haven’t found anything.