Re: The problem
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?
I've found that it's not the number of tabs open, but the types of websites and how many you browse. For example, if you visit and re-visit the awful and bloated Walmart website often enough in just one tab, your RAM goes to 1.8G and higher, and browsing on any tab becomes painfully slow.
On Firefox, the more RAM a profile consumes, the slower that profile gets until you are forced to re-start the browser.