I'd suggest Waterfox.
A lot of Firefox addons don't work in Pale Moon, including a significant number of the legacy addons as well as all of the Webextensions-based addons. PM is based on a much older FF build (pre-Australis), without e10s or any of the performance boosts Mozilla has made to Firefox in the last few releases leading up to FF 57. Not having Australis is a good thing (to many of us, at least), but it would be nice to have e10s and the other optimizations.
Waterfox has the controversial Australis UI, but it also continues to support Classic Theme Restorer, so Australis is easily eliminated. Waterfox supports more Firefox addons than Pale Moon or the current Firefox (as did Firefox 56 too). It's essentially FF 56 ESR, if there was such a thing, with a few changes (pocket removed, telemetry removed, NPAPI plugins and unsigned themes still work if you want them to, stuff like that).
I tested FF 57 and WF 56 on the same PC (i5-2500k desktop) with the same settings, with my full complement of addons. WF 56 had 23 addons enabled, while FF 57 only had about 8, since that was all of them that still worked or had Mozilla-suggested replacements at that moment. Even with three times the addons, nearly all of them of the supposedly performance-robbing legacy variety, WF 56 only scored 8.5% slower than FF 57 on the Speedometer 1.0 browser benchmark (Speedometer is the one Mozilla itself uses to tout how fast Quantum is supposed to be). I got essentially identical results in Windows 8.1 and Linux Mint 18.2.
Mozilla, clearly, did not need to eliminate the powerful "legacy" addon APIs to get a big speed boost from Firefox. That's the idea they're trying to pitch with their "twice as fast" ad campaign to try to justify removing Firefox's defining feature, but nearly all of that gain was from changes that were made prior to the release of FF Quantum. The powerful addons didn't have to die to make Firefox fast.
By contrast, Pale Moon (same setup as in the other tests, with and without addons disabled) scored 40% slower than FF 57.
The actual results were: FF 57, 88.0; WF 56, 81.0; PM 27.6.1, 53.07.