On my desktop (using Chrome), I have about 50 tabs open, which is apparently using about 3 gigs of memory (EDIT: and certainly even more of the address space when you count memory mapped files). Granted, I have heard that Firefox uses less memory than Chromium these days, due to the multi-process overhead, but nonetheless I imagine those numbers would be somewhat similar on Firefox.
Thousands of tabs may be a fringe use case, but clearly, you can approach the 32-bit address space limit under much more sensible loads.
My experience with 50 tabs open in Firefox and Chrome is that Chrome uses about 75% more memory. You can test this yourself instead of guessing. Also, people with 50 or more tabs open represent less than 1/4th of one percent of Firefox users and I'd imagine that's roughly the same for Chrome and IE so I think it's a stretch to call that a "sensible load". 50 tabs is an extreme load and Firefox handles it better than Chrome in my experience.
Thousands of tabs may be a fringe use case, but clearly, you can approach the 32-bit address space limit under much more sensible loads.