I switched away from firefox once they announced they were moving to the Chrome extension model (and thereby sank their "better extensions" selling point.)
Sure. But in practice everyone is just going to make extensions that are compatible with "all" the browsers, and there are developers who have simply decided not to rewrite their firefox extensions.
There's a Tree Style Tabs-like WebExtension. It has a few quirks, though. It runs in the sidebar. It doesn't hide the top tabs (though you can do this with Stylish). It has its own menu for tabs which does not include some options (like send tab to device).
There was a very good non-nested side-tabs extension in Firefox Test Pilot, but it has expired.
Our long-term plans for Pale Moon involve (potentially) moving our browser to the UXP (Unified XUL Platform) that is currently being worked on alongside the browser. This will at its earliest be somewhere in 2018.
Plug-ins
Pale Moon supports NPAPI plug-ins. Unlike Firefox, we will not be deprecating or removing support for these kinds of plug-ins. This means that you will be able to continue using your media, authentication, and other plug-ins in Pale Moon like Flash, Silverlight, bank-authenticators or networking plug-ins for specific purposes.