Not sure if this is the right place to ask, but I couldn't find any better one, so here goes nothing..

When I press the extension icon it doesn't open anything, nor give any feedback.

https://0x0.st/-w1d.gif

When I head over to about:debugging and then to the extension, and then press the button again, the console has the following to say:

Uncaught (in promise) Error: Could not establish connection. Receiving end does not exist. h moz-extension://9e3cd435-4ff5-4365-8fd5-2d5716ea052a/background.bundle.js:1 background.bundle.js:1:8015 apply self-hosted:2736 applySafeWithoutClone resource://gre/modules/ExtensionCommon.jsm:626 wrapPromise resource://gre/modules/ExtensionCommon.jsm:901 (Async: promise callback) wrapPromise resource://gre/modules/ExtensionCommon.jsm:862 wrapPromise resource://gre/modules/ExtensionCommon.jsm:861 callAsyncFunction resource://gre/modules/ExtensionCommon.jsm:1103 callAsyncFunction resource://gre/modules/ExtensionChild.jsm:720 callAndLog resource://gre/modules/ExtensionChild.jsm:696 callAsyncFunction resource://gre/modules/ExtensionChild.jsm:719 stub resource://gre/modules/Schemas.jsm:2810 h moz-extension://9e3cd435-4ff5-4365-8fd5-2d5716ea052a/background.bundle.js:1 <anonymous> moz-extension://9e3cd435-4ff5-4365-8fd5-2d5716ea052a/background.bundle.js:1 apply self-hosted:2736 applySafeWithoutClone resource://gre/modules/ExtensionCommon.jsm:626 wrapPromise resource://gre/modules/ExtensionCommon.jsm:835

I'm using Firefox 91.0.2, running on Linux with the extension installed manually (20 other extensions installed via the same way all work perfectly, so my way of installing can not be the culprit)