I've tried Firefox limited to 1 GB for a laugh. It's usable. It won't do many tabs at the same time but it's usable.
You can actually go lower than that but you'll start to run into limitations with YouTube videos etc.
There are also other browsers out there that are more light-weight but perhaps not as feature-full as Firefox. Giving up extensions alone reduces a lot of complexity. If you fire up the package installer on any Linux distro and search for "browser" you'll find a ton. There aren't many engines but there are a lot of browsers.
With cgroups, it's a standard kernel feature. You can limit RAM, CPU, network access, lots of things. It's used in Docker, LXC, Kubernetes and lots of container solutions.
There's Linux dists that can only requires less than 200 MB of RAM. Absolute Linux for an example, has a minimum system requirement of 64 MB RAM. Plenty of space left for memory hungry softwares like a browser.
I've tried to use Fedora Workstation in VM (GNOME Boxes) with only 1GiB RAM. And it is even usable and UI is responsible for GNOME and Firefox, but applications start more slowly. All those at cost of higher CPU usage. Probably it performs well because Fedora uses swap on ZRam, and it makes the system more reliable.