The browser extension version is very handy, because it is always right there in your favourite browser. You can get it from the Mozilla, Chrome and Edge add-on stores.
Kiwix JS Windows/Linux is a more fully featured release customized for the following platforms: Electron (Win 7/8/10/11 and Ubuntu / Debian / OpenSUSE, 32bit and 64bit); NWJS (Win XP/Vista/7/8/10/11 32bit and 64bit), UWP (Win 10/11) and an installable PWA. There are portable and self-updating installable versions. The universal PWA version can be used instantly just by visiting https://pwa.kiwix.org (in Chrome and Edge, you can install it as a fully fledged app, see install button on Configuration page). It’s always up to date and works offline after you install it!
There are also English WikiMed and Wikivoyage versions of this app which come packaged with the respective ZIM archives.
Please note these API limitations: the browser extension version requires you to pick the ZIM archive you wish to use each time you open the app (or you can drag-and-drop the ZIM into the extension). The PWA version uses the File System Access API to provide a near-native experience in Chrome/Edge, but this API is not supported by Firefox. The Electron/NWJS and UWP versions have file system access natively.
Changelogs for these releases are: Kiwix JS browser extension and Kiwix JS Windows/Linux.