That's some really weird CPU and memory use... TBH I'd expect that from a site using crypt-mining scripts. If you're running web apps, it can push your usage up, but even then, you'd have to overload it with 1.21 Gigatabs (film ref.)
In all seriousness, if you haven't tried doing a full reinstall (you can export your bookmarks and import them) then I suggest you do. Export your bookmarks to you home dir, purge FF and the config dirs and do a fresh install. You can then import your bookmarks and you'll have a clean slate and default settings. Alternatively, you may want to use an upstream AppImage of FF just to see if the fault is the version you're running.
There are some other considerations:
- Heavy theming + loads of extensions/plugins, etc.
- Erroneous scripts (incl. crypto-miners) on the sites you're visiting. Installing NoScript/Ublock helps.
- You're on an iGPU which means anything graphical like font AA and smooth scrolling to graphics acceleration will dent it.
You could try other browsers, too, as I suspect it's likely a glitchy config/version:
- Librewolf which is literally FF with a few privacy-enhancing changes and Ublock preinstalled (which may reduce the footprint for reasons of the above and the disabling of some FF "features"). I'm posting from it now (though I swapped out Ublock for NoScript as it's easier).
- Midori/Floorp/etc. browsers. All more/less the same, have their roots in FF, work well, render correctly, and are very fast and snappy. Thay're very lightweight.
One thing I can say is there's no way that machine of yours is too low specc'd... unless it's CPU optimizations like h/w decoding for newer toys like HEVC/x265 and AV1 vis a vis straming video, or AVX2 vis a vis some weird browser game (if browsers have come that far).