This is a follow-on to https://support.mozilla.org/en-US/questions/1401783, I have the same problem - I've followed https://support.mozilla.org/en-US/kb/extended-add-support (which is for some bizarre reason marked "no longer maintained"), to create the custom collection https://addons.mozilla.org/en-US/firefox/collections/17515772/raxod502/ so that I can override the arbitrary blocking of third-party extensions that Mozilla applies by default. This allows me to install most extensions but for some reason https://addons.mozilla.org/en-US/firefox/addon/duochrome/ is not showing up in the interface on Firefox for Android, despite being in the custom collection. No information is given about why it is not showing up. I am not aware of any reason why it would be blocked. Here's the manifest.json: https://github.com/FreshSupaSulley/Duochrome/blob/main/firefox/manifest.json How can I fix this and use whatever extensions I want to use, or at least get a valid error message about why Mozilla is blocking the extension? Creating a new thread since the older one just says "Even using that approach, not all add-ons are supported in Firefox for Android at the present time", which doesn't help at all - there is no reason why any addon should be blocked for mobile usage unless it is explicitly configured as such, and even if that were reasonable, it doesn't help explain what specifically is wrong with the extension I'm trying to install. More context: https://old.reddit.com/r/firefox/comments/vdsnrz/all_the_extensions_on_firefox_android/icq2itt/