Re: The problem
"FF42 is using 1.3G and it gets worse and worse as you keep browsing."
I just checked Process Explorer, and what do you know - 1.1GB. That sounds pretty bad, but bear in mind I have 2 Firefox windows open with several hundred tabs (though only a small subset are loaded). I wonder if Mozilla is trying to boost performance by using more RAM?
"It's almost like the modern versions of FF act like old Windows 98, using all all available RAM and getting slower and slower until you have to restart it to get it running fast again."
Using all 'available' RAM is not in itself a bad strategy - empty RAM is wasted RAM. Also, RAM gets cheaper all the time. I have 16GB in my current system, so if Firefox could run like the blazes by using 1, 2 or 3GB, that might be a pretty good deal.
But the real question is efficiency, of both code execution and memory use. Unfortunately, I suspect Firefox (like most browsers, including Edge) is being optimized to deliver blistering benchmarks when loading a single page. There's probably room for huge improvements in handling multiple tabs and multiple extensions.