install extensions and open lots of tabs then there may be not enough free memory firefox is very usefull with extensions. cant browse without it. also i have tried 64bit beta firefox. it is buggy. i couldnt use it.
Waterfox I haven't tried any of the other 64-bit variants (Palemoon, Cyberfox) so I can't recommend them. I had Waterfox installed for a while, but went back to using Firefox 32-bit.
64bit firefox is slower than 32bit firefox in browser tests. fwaterfox and cyberfox are all based on the same firefox core.