FF Apps Archive contains thousands of apps from the defunct Firefox Marketplace.
Packaged Apps - can be installed here
Privileged Apps - cannot be installed directly and must be sideloaded using developer tools
Live Hosted Apps - can be installed, but might no longer function
Dead Hosted Apps - cannot be installed
Firefox apps can be installed on any version of Firefox OS and on old versions of the Firefox web browser v16 - v46.
Firefox desktop v45
Firefox Android v45
Firefox OS 2.5 Android launcher
FFAA is an open source project licensed under MPLv2. Contributions are welcome.