The Pale Moon forums have a post titled " How to install uBlock Origin in Pale Moon 27 revised". A lot of extensions you may be familiar with are not listed in that catalog. Old XUL/XPCOM Firefox extensions and updated versions of them still using XUL/XPCOM, if they exist, will mostly work with Pale Moon.
All modern Firefox extensions are developed using their new WebExtensions API. The move rendered all existing Firefox extensions useless. Mozilla switched from XUL/XPCOM extensions to a new extension API/format called " WebExtensions" in 2017. The first thing which comes to mind after using it for a few minutes is therefore likely "How do I install Ublock Origin"? Pale Moon does not come with any built-in advertisement/garbage filter. Pages using those technologies will not work as you would expect. Pale Moon does not support WebAssemly or WebRTC. WebGL pages render fine, JavaScript intensive pages are fine but there are some limitations. Pages load fast and they render as you would expect. Pale Moon generally fine for causal browsing. Pale Moon has made improvements to the interface since they forked from Firefox in 2009, it is not identical but it does feel identical. The preferences dialog is almost identical to the one Firefox had before they "modernized" it. A separate search-field to the right of the address bar, which is by default gone in Firefox (it can still be enabled), is shown by default and so is the useful menu line modern Firefox hides in favor of a smartphone hamburger-bar. The bottom status-bar Firefox had back then is still intact in Pale Moon. The interface is what Firefox 24 had with several small changes. Those who have used the Firefox web browser for a very long time will immediately recognize Pale Moon as a very old Firefox version.