It's not. Firefox 52 based browsers suffer with memory leak hell.
Also, regarding FailMoon, I don't understand the love for it as a product when SeaMonkey has existed for much longer and has much better natured developers that may actually be able to get things done in a much more civil manner with much less infighting and drama, much better PR and overall a much more polished and stable product that while might not be so good for 'XP-era' systems due to the 2.53 line being incompatible, they're literally backporting things from the latest Firefox ESR onto FF56/60's engine... not to mention that the SeaMonkey devs were forcefully kicked out of their funding from Mozilla and forced to look elsewhere, something that has been ongoing since the past four or five years now or so, while FailMoon has not had to deal with being screwed over themselves other than supposedly by their own narcissistic reasons to make up drama much of the time.
Oh, also, I always knew the PM drama would happen with RT1/Feodor2 eventually and that it was only a matter of time. They've been pulling stupid things for a while like publicly denying support for people due to what Linux distro they use (Slackware in that case), the whole fiasco with the OpenBSD install script, the NoScript block in the browser, refusing to implement things while claiming to follow the standard "because they don't agree with it on a political level"*, among other things. There was something else too but I forgot what.
* Guess what, web standards exist and they are to be followed generally speaking, not so much when it comes to 'design trends' or anything as that's an artistic thing anyway - ignoring those is fine, but anything that's an actual specification or whatever should not be followed liberally, so to say. Like, I dunno, take a packet of crisps for instance. Not every brand follows the same colours on the packets (glares at Walkers) but they always have the same formatting for the nutritional information and stuff; one is a design trend, and one is a standard.
(Fun fact, the developer of the Mega Drive demo 'CrazyBus' - yup, that one with the loud, randomly generated PSG tones for the title screen - actually uses SeaMonkey exclusively).