Classic Add-Ons Archive
Catalog of classic Firefox add-ons created before WebExtensions apocalypse
3 Alternatives To Classic Add-Ons Archive
Addoncrop
Extends the functionality of your web browser
F-Droid
F-Droid is an interesting alternative for Android users who want to try something different and not have to use the Goole Play store all the time.
Yalp Store
Yalp Store lets you download apps from Google Play Store as apk files.