I used the 64 bit version of Waterfox for a while when Firefox still didn't have a 64 bit version (I keep a lot of tabs open and the 32 bit version of Firefox would quickly hit the 2 GB limit of 32 bit applications).
Went back to Firefox when the 64 bit version was released as it solved the running out of memory problem (Mozilla is constantly on top of security updates, etc, so my guess is that it is much safer than Waterfox, but feel free to correct me if I am wrong) but it created another issue: memory fragmentation. Eventually memory would become so fragmented that Firefox 64 bit would become slow as molasses.
Opening the About:Performance page (IIRC) and compacting memory fixed the issue for a little while, but the slowness would eventually come back, so it was a bit of a PITA. Considered switching to Chrome for a while, but then the Quantum version of Firefox was released and I have been happy ever since.
Of course, Firefox 64 bit will STILL swallow up all your RAM if you let it, but at least now it remains snappy while doing so.