I think the thing people are questioning is the "It's when I open a lot of tabs" comment.
Here's the thing. It may not be. I know this because every now and again I too prune the tabs. And for a while, it looks like that's solved the problem. Only the memory issues suddenly flare up again. And usually they do when I open lots more tabs... but sometimes not.
After a long while I tried using about:memory and looking at the reports it gave. This showed me that many web pages I'd assumed were problematic, for example Twitter, GMail, and the modern AJAX-heavy Google home page, weren't actually that bad. Well, they're bad, they use tens of megabytes for something ridiculously simple (consider the fact the Mozilla binary is smaller than the footprint of many of these pages and you'll see where I'm coming from) but they're not the cause of the "Uh-oh, Firefox is now using 2G of RAM and is about to crash" thing.
On the other hand I found that a hobbyist website I frequent whose design appeared to have not changed in ten years beyond the owners adding the requisite "Share via 600 social media networks!", "Here's which of our articles are trending on Faceplace!", etc, add-on JS libraries, was eating hundreds of megabytes across a small handful of tabs. I, of course, would visit that website, go to the news page, open three or four articles in new tabs, and leave them open without thinking about it because I was monitoring the discussion, and didn't realize the impact.
Is this Firefox's fault? No idea. There's a few things that can be going on: it could be that the pages are using JS in a way that's impossible to sanely garbage collect (ie simple bugs that result in references to giant supposed-to-be-temporary objects remaining linked to), it could be some obscure bug in Firefox resulting in memory leakage (less likely, simply because I know they've been working on it, and I know that Chrome also shows the same websites apparently using more memory, it could be that Firefox's JS implementation is inefficient and uses more memory per object than is strictly necessary, or a whole host of other causes.
Me? I'm thinking Firefox could probably help with solving the issue by isolating tabs but also by punishing webpages that start to go out of control like this. We already have a situation where a CPU heavy script results in an alert to the user (albeit a not very well implemented one - I don't want a modal dialog coming up about a tab I'm not looking at thankyouverymuch, attach it to the tab in question, change the color of the tab, but let me finish what I'm reading in the tab I have in front please.) Perhaps a similar dialog needs to come up when a tab starts to exceed a particular limit on memory, with a "Continue (Doubling memory limit for tab), Stop script, Reload tab" option set.