Take this with a grain of salt. I'm not an expert, and I'm a Windows user (Though I've used Linux some in the past). So, I'm probably wrong. However, I saw a question raised that wasn't answered, and thought I'd take a (Likely inaccurate) stab at it.
Earlier in the thread, there was a question raised as to which distro would be the best for development and testing against.
I note that on DistroWatch, the top three distros by users are listed as being Mint, Debian, and Ubuntu. Ubuntu is a fork of Debian, and Mint is a fork of Ubuntu (normally) or Debian (special edition available to users who prefer it). Even lower down, I noticed that in the 7 and 8 positions (Elementary and Zorin) are two distros listed on the site as being in some way based on Debian and/or Ubuntu, and I would suspect I'd find more if I kept clicking around.
So, given that Mint, Ubuntu, and Debian are very similar "under the hood" and closely tied together, and are the three most popular distros, it seems to a lay person to make sense to concentrate development efforts on being broadly compatible with things in the .deb neighborhood, Linux wise.
Of course, if one had a browser that was specifically geared towards getting business IT departments to adopt it, given the penetration of Red Hat into the business world, CentOS (Which sticks close to Red Hat on delay and without the copyrighted components) would be the logical choice to develop for. My impression, though, is that Pale Moon primarily is going after home users and/or users who may use their computer for business purposes, but essentially home users in the sense of having control over their own device and aren't managing or subject to an IT department. Clearly, the Debian-derived distros are dominating desktop user share in general, and especially in the home user segment, I'd imagine.
However, that doesn't mean that CentOS is a bad environment to develop in and for. Maybe it's an easier distro to develop on in order to create the largest possible cross-compatibility or something, perhaps precisely because of the older packages it uses. I have no clue.
I noticed there is a guy doing an official Pale Moon for Ubuntu package in an "unofficial" (i.e. Not directly authorized or managed by Ubuntu) repository he hosts himself, and he describes being largely able to do it with somewhat automated software based on the generic tarball or whatever that Pale Moon puts out. So, developing on CentOS doesn't seem to really be hindering compatibility with the distros that are most popular, probably negating an impetus to abandon CentOS for them if the developer(s) are most comfortable using CentOS.
Really, I think the key thing I'd be looking at if I were involved with a browser trying to get more Linux users, is how to get the browser pre-installed as the default browser on some distros, and if not, how to get people to put in the official repositories and stores where it'll come up on top when users search for browsers other than Firefox and Chromium, with devs from the distros maintaining a compatible version optimized for their distros concurrent with each release from Pale Moon itself, the way they do with Firefox.
However, how much it's really possible to get that kind of placement and effort from the distroes, and, if it's possible, how one would go about it, and whether or not it's worth the time and resources that might be involved in making it happening- no clue.
But I do think it'd obviously be good for Pale Moon if somehow people installed Mint or Ubuntu or something and by default, Pale Moon was the web browser sitting on their desktop instead of Firefox. If Pale Moon were a corporate-backed project, I'd say try to buy that placement. Since it's not... I wonder if there are listservs or something where the top devs on the top distros could be asked why Firefox over Pale Moon, and so forth. Pale Moon seems closer to Linux's ideals.
Of course, really, when Linux desktops as a whole are only at 1% or so of the market, and have been stuck there for a long time, making too much of an effort to fight over how much of the 1% Pale Moon can be popular on isn't necessarily an efficient use of time and resources relative to battling for more Windows marketshare, or even developing a Mac version, or restarting Android development or whatever- unless it's just something that the people involved with the project want to do because they like Linux.
Just some thoughts. Probably ill-informed. Feel free to ignore them if they don't make sense.
If you can find anything in there that's useful, though, I'm happy I was able to help a little bit.