Pale Moon, & Waterfox are built on an older more customization style of Firefox. So although not identical, they're all almost the same thing in many ways. Pale Moon & Waterfox are what we used to love about Firefox years ago, before Firefox decide it wanted to imitate Google Chrome.
Why would we want a browser that tries to imitate Chrome when Chrome already exists if we want it? So they lost a lot of market share to Chrome just by trying to imitate them. It would be like all Vanilla makers deciding to abandon making vanilla & make imitation chocolate instead. If we're forced to choose between only imitation chocolate or real chocolate because nobody would offer Vanilla anymore, we'd choose the real stuff rather than the imitation, or maybe just switch to strawberry or banana flavor if they refuse to offer vanilla anymore.
Microsoft made the same mistake with Windows 8-10. They decided that their OS should imitate android & iphones & abandon their popular desktop PC format. Most people aren't very computer savvy. If Microsoft is gonna make their computer hard for them, or make them imitate phones, your average person will just walk away from computers & just use their phones or an android tablet instead of something that tries to poorly imitate them.
Most people aren't building websites, producing or editing media, doing presentations, or crunching numbers on a computer. They just need something to get on Facebook with, get email, shop at Amazon, check their bank with, & communicate with contacts with.
So while Microsoft is patting themselves on their backs for still having most the desktop market share, what they failed to notice is because of their poor decisions to force stuff that we don't want on people, that people have abandoned their real computers or desktop for simpler, easier, & cheaper phones & tablets in masses. So there's less people on real computers now than there has been since the mid 1990s.
A simple glance at the stats for Betty's & most websites show that more than half their visitors are visiting on android phones or tablets, & are rarely or never touching a computer anymore thanks to Microsoft's Windows 8-10 shenanigans. And Apple products are way overpriced, but stopped being premium quality devices over a decade ago.
As far as who has the market share of who's online in the USA & wordwide, stats indicate it is android, with more people online with Samsung portable devices than anything else.
Anywho, back to the original topic. I use Pale Moon for almost all my general web surfing. It's faster, more secure, & less bloated for general surfing than the rest. But none of the browsers are perfect. For one thing the owner creator of the group behind Pale Moon is a total jerk. He may ban you from the Pale Moon site & help forum for asking a simple question about a Pale Moon flaw or defect.
He feels everyone there should have absolute praise for their browser. Anyone who hints that there might be a problem with the browser gets immediately banned. LOL, he had no idea who he was dealing with when he did it to me for asking about memory leaks & high CPU usage while on Facebook. I got back in within minute of being banned, just to ask him why the hell did he ban me for asking a simple, plain, technical question. He banned me again without replying & deleted my question.
So "someone" hacked & screwed up his site after that. It took him a couple days to straighten out the mess. I teased him, not to ban professionals asking reasonable professional questions, or they'll show you how pro they really are.
All browsers have the flaws, or conflict with what I want to do. Pale Moon has some memory leaks & higher CPU usage on Facebook, Google Drive, or when working on my YouTube channels. My bank doesn't like Pale Moon. Everything works there except when I try to get at my credit card statements, or try to pay my credit card bill.
So for those things I use Firefox. It works better for those things for me. But at other sites Firefox gets memory leaks, high RAM & CPU usage. It can get quite sluggish & bloated at overloaded sites... even if I spend too much time of Facebook it would start to get really bogged down.
I use Waterfox as my browser to install tons of plug-ins, & extensions to for all sorts of cool features & hack tricks. I don't want to bog down my general browser with all that stuff, so most of my bag of tricks are installed on Waterfox. It's handy because it's still compatable with a lot of cool or older add-ons, that are no longer compatible with Firefox, that I couldn't work without. I even use Waterfox to download & save video off facebook, news sites, & other sites where you can't technically download & save their video or audio.
Basically anywhere I go where I need some hack trick to get something to work the way I want it to work instead how they want it to work, or get something I want, I use Waterfox. Waterfox is my hack tool. Most of my hack tricks are done through Waterfox.
The great thing about all 3 is they have a lot of the same basic programming so my browser user profiles are compatible with all of them. I can copy my user profile from Pale Moon, & overwrite it into the profiles of the Firefox or Waterfox. So basically I can transfer all my bookmarks, passwords, cookies, & preferences, to any of the other browsers. No more lost bookmarks & passwords. I can take that profile on a thumb drive with all that data & put it on my laptop or other computer with Firefox, Pale Moon, or Waterfox on it... even Linux computers.
So all my computers & laptops have my same online passwords, bookmarks, & preferences.
I avoid Google Chrome. It uses too much of my internet connection's bandwidth on it's telemetry/spying like Windows 10 does, but not quite as bad as W10. I'm on slow DSL, so can't afford to have anything consuming my connection for no good reason. Chrome also installs lots of bits of itself all over the computer. A browser should be stand-alone, not infest & embed itself all over inside the computer & attach itself to other programs where it has no business going. It even installs bits of itself into your other browsers without your knowledge of permission.
I believe it's very bad for any browser to install bits or itself on other browsers in your machine. It shouldn't be touching any other browsers on a machine. It can take hours to manually get every bit of Google Chrome out of your computer if you installed it. Normal "uninstall" does not uninstall all of it, just a small bit of it.
I used to like Opera. I liked to use their turbo mode as a poor man's proxy or VPN, or to speed up overloaded sites. It was great & streamlined for phones & android tablets too. But it's gotten weird, slower, & hard to use lately so I don't use it anymore. Pale Moon doesn't do well on android. So I'm using only Firefox on the android stuff.
I try & test just about every browser out there a couple times a year. I actually earn a little bit of money writing reviews about them too, so really have to study them. That's how I learned to keep more than 1 browser on a machine, & know all the tricks & drawbacks they all have.