I started a new article titled Firefox User Interface Customization which is a work in progress. At this point there is only a few of CSS code samples for customizing Firefox and its derivatives, but i expect it will grow to include a lot more. The code will work with Waterfox as well, which i know because that’s what i’m currently running.
The transition from XUL to WebExtensions for Firefox add-ons is not going smoothly at all in my case and this is one reason i ditched Firefox and started running Waterfox, the latter of which is a bit more focused on privacy as well as continued support for XUL (legacy) extensions of which i have several that are simply not replaceable at this point. I think the biggest problem with the move to WebExtensions is the limitations in the API and i think it’s a [yet another] mistake by Mozilla to drop support for legacy extensions with the upcoming version of Firefox 57 given that their API seems to be only half-baked. I might have tried Pale Moon, except they apparently have no plans to support WebExtensions whatsoever. I suspect this might come back to bite them in the ass since i’m sure that the vast majority of Moon users get their add-ons from Mozilla and many of those developers seem to be abandoning the legacy extensions that Pale Moon depends on.