What you should do now:
First try a different browser (preferrably brave) and see if it has the same problem.
If it has the same problem: file a bug against pulseaudio
If it works flawlessly: file a bug against firefox
If you want to attack the problem yourself there are few options:
You said earlier that you couldn't see the priority or niceness of the pulsaudio process so let's look at that first because if you're going to experiment it is highly recommmended that you can check the priority and niceness constantly. By default the process manager doesn't show these columns so you have to enable them in the settings.
(personally I prefer to use 'htop' for that, also because it works better)
Now you can check if '--high-priority=yes' did work.
If it didn't, this should work:
sudo renice -n -19 pulseaudio
If this is the only way for you to boost the priority, then it would involve a bit of reasonably complex scripting (with IPC) to automate this at logintime.
Perhaps it would be better to lower the priority of firefox instead of raise the priority of pulse.
It would also make any remidies a lot simpeler....can you try this first ??
The other way is to let pulse restart automatically a few seconds after you start firefox.
If this all doesn't work, you could configure pulseaudio to start systemwide instead of as a user instance. It will get the UID of the system and therefore a higher privilege and the possibility to easily elevate the priority. It is not for the faint of heart and you are completely on your own if you try this (and it might break with an update) but as a last resort it could be the solution for you. At least know that pulseaudio is designed to be able to run both systemwide or as a user instance, depending on your preference.
About the strange behaviour( like restarting pulse or firefox to temporary solve the problem):
Since you have an FX processor you have 8 integer cores and 4 floating point cores, which is very roughly comparable but still a completely different beast than 4 hyperthreaded or 8 non-hyperthreaded cores and can sometimes in very special and obscure cases behave also a little bit different than planned.
If you (temporarily) disable every odd core and the problem disappears then you know it's an architectural problem.
Still you must file a bug because although you have an FX-processor, the behaviour is still unacceptable 