Start Firefox, 89%+ Utilization.
Current version of Firefox is unusable.
you can't use Bank web-sites, GMail, .. News sites with never-ending stream of news.
Firefox goes crazy and hangs.
Excuse me?

!!!!
Now now...GMail user here for years now...and running quite fine.
....but...as Dave mentioned, you need to prep for this: NoScript helps (but has it's own downsides), uBlock helps as well.
So what specifically do you have problems with re: gmail access?
To the OP (Dan):
There are some things our FF sturggles with, and as Dave already mentioned, some of them can literally peg your CPU to 100% (jscript is perfect example of one such 'bad actor'). I find that a SMP box can actually mange this well and in cases when this does happen there are plenty of remaining CPU cycles to carry on happily.
Keep in mind, I base this on my experience with a 6-core AMD Phenom II X6 CPU and my current AMD FX8370 which is a 8-core CPU. Even though the FX8370 is actually a weaker of the two CPUs, the two additional cores more than make up for what used to bring my PhenomII down completely.
So there are some FF profile settings which will enable you to deal with some of this.
Take a look at:
- content.interrupt.parsing
- content.max.tokenizing.time
- content.maxtextrun
- content.notify.backoffcount
- content.notify.interval
- content.notify.ontimer
- content.siwtch.threshold
All of the above control how FF responds to the updates and the frequency of rendering these updates to the screen.
Beyond these, you have the whole 'network.http.*' bucket which will control your keep-alive stuff, the connections-per-server, pipelining, etc., which when set accordingly to how you connect to the Internet can make a difference.
For what it's worth: if someone can tell me how to extract my settings I'm happy to u/l them here for further review. I think I can just get the flat text from one of the profile files too...let me know!