I once ran an x64 Firefox on my PC, with 3GB RAM and 2GB swap (that was from a live USB, swap was two partitions on two different hard drives). It was thus able to take over every system resource. (I have a bit of a tab hoarding problem but sometimes the sites are extremely heavy). Linux behaves "interestingly" when you run out of swap, I'll maybe call it "meta-swapping". The mouse cursor gets freezing but somehow it still works. Killing plugin-container is useful when you manage to do it.
So, Firefox 32bit is almost a feature. Note that 32bit Chromium is able to use all my memory and swap, and does it earlier.
My bro has a PC with crazy RAM, 32GB (a store once fucked up on pricing). He loads about a 100 tabs or two and then, Chrome is using 10GB memory.