A reasonable web browsing experience is no longer possible (on any browser) on machines with 8 GB of RAM or less. Horrific, but true. Either get more RAM, use something like Total Suspender to suspend inactive tabs to disk, or limit yourself to 10-15 tabs open at once (and fewer than that if you're using modern, heavy JS web applications).
Dunno man, I'm actually using it right now and with 4 tabs open it's using 1.3 gb of RAM. These websites aren't specially heavy either, none of them it's Facebook or another social network.
My understanding is that the RAM per tab metric drops as you have more tabs. Firefox is more willing to cache stuff when you have only 4 tabs open, so that's not a great metric. Plus you have to consider the base RAM use of the browser itself. 1.3 GB isn't all that much on a modern system with 8 GB available, for example; the problem is more that Firefox doesn't handle 20+ tabs very well especially when you've got Twitter, Google Docs, Slack, etc open.
My recent experience of keeping 1000 tabs around and auto unloaded (managed by Sidebery, a vertical tab bar plugin), is that it usually occupies 3~5 GBs.
