I've done all the tweaks and configurations found online, and nothing has made a marked improvement whatsoever. Not going to switch, but it's incredibly frustrating.
I had support temporarily disable McAfee's "on-access scanner" thing so I could test it and the problem went away immediately. Everything was nice and snappy again!
Of course, they turned it back on after the test...
I guess maybe it could be Windows Defender? Doesn't quite explain why the Firefox process is the one that is sucking literally my entire r/w bandwidth, slowing my computer to a crawl and inhibiting all OS functions. Defender does this every once in a while, randomly, but it doesn't show up during Firefox's start-up.
It works fine after like a minute of warm-up, but I still think that's unacceptable on an SSD. Still gonna use Firefox of course...