My experience is exactly the opposite. Each time I'm past 200 tabs or something (i.e. all the time), Chrome's UI starts occasionally lagging and glitching, and I'm not even talking about memory usage (overall or even just parent process alone) and how it's affecting the operating system. Even with pre-Quantum Firefox I was always pushing limits much, much further.

However I'm avoiding media- and script-heavy websites as much as possible, and block a couple of ad networks solely because of the stress that rich media ads (videos flying all over iframes and whatnot) causes for the hardware I'm running the browser on. (I don't mind ads if they don't take 8 CPU cores to render, but they seem to be disappearing from the internet.)

That's weird. I currently have ~100 tabs open in Firefox (yes, on the same computer as 800 in Chrome), and it's fine. Back in 2008, I had ~800 tabs in Firefox on Linux for a project (with TabMixPlus); it was pretty slow and unstable, but it worked.