This seems to have happened over several updates, so I can't pin point a particular update, and the problem has gotten worse (or at least affected more things) as time ha… (emoñe’ẽve)
This seems to have happened over several updates, so I can't pin point a particular update, and the problem has gotten worse (or at least affected more things) as time has gone on. I thought it might be related to uploading tabs when memory utilization has gotten high, but I have this problem on my system with 96GB of RAM, and I've happens even when I'm using under 48GB of memory and I've tried disabling tab unloading with no change.
The problem I've been seeing first started with Skype web. I'd walk away from my PC, and when I get back to it (monitor is asleep, but PC still running) after I wake up the monitor Skype would start playing messages it appeared to have cached while I was away. It would load up this messages one at a time, complete with notifications, one every couple of seconds. If I'd been away for a while and had gotten several messages it could take several minutes to catch up with where Skype is "current" and usually take so long I end up just reloading the page and so I get the current state without waiting for FF to play through all the messages.
I then started noticing the problem with OctoPrint, similar thing I'll have it up monitoring a print and I'll walk away from the PC and then come back, and same thing, it has to "play" through all the updates that happened while I was way from the PC, again sometimes taking several minutes for it to catch up with the current state for OctoPrint (this is usually visible by watching the temperature graph try and draw updates to catch up with where the printer actually is. During this time OctoPrint is completely unresponsive, so I'm unable to do things like cancel and print or change tabs to look at say the web cam. Again I'm usually forced to just reload the page to get the current state of OctoPrint instead of waiting minutes for it to catch up.
Then recently it's started doing the same thing with Pushbullet, instead of sending notifications as they come if I'm away from the PC, it just caches them when I wake the screen back up. And again, it sits there and plays them, one at a time, every couple seconds, every, last, message, that, came, while, I, was, away, ding, ding, ding, ding, ding. I'm no longer getting multiple notifications at a time, they come through individually and it drives me insane (at sometimes delays notifications as even when I'm using the PC it no longer delivers notifications if I'm not actively using FF).
I assume it's some change to help make FF less memory intensive (which is why I first started looking at Tab Unloading) but I haven't had any luck finding out what's causing the issue, so trying to Google for a solution hasn't yielded any results (cause I'm not really sure what I'm looking for).
It's making using FireFox for "live" pages like message apps and OctoPrint, and notification syncing via Pushbullet unsustainable.