Extensions for desktop Firefox (for Windows, Mac, Linux) from addons.mozilla.org/firefox are not really 32-bit or 64-bit. A small number of Extensions may be OS specific due to using something specific on OS for example. Only one NPAPI Plugin is allowed to run in Firefox since the 52.0 Release and that is Flash Player from Adobe. The word Addons started back during the Firefox 2.0/3.0 days and is a general term that groups together the separate Extensions, Themes, Plugins, Dictionaries, Language Packs, Search Engines. Oh and Facebook uses a HTML5 player for video/audio and not the Flash Plugin. For games it may use Flash.