I've updated what looks like a new required "data_collection_permissions" permission that Firefox added, so this might resolve this issue. However, I do not currently have access to a Linux machine to test, so I am uncertain, but this is working on the Windows version of Firefox.

Also, to be clear the message that says "Can't read and change data on this site" is correct and expected. This addon does not require any access to site data, the only permission is to tab data so that the url can be read.

The intended behavior of the addon is when you are on the correct site, it will add an icon to the url bar that can be clicked to toggle the UI, you can see the screenshots for examples. (also, I've now added a keyboard shortcut that can be used to perform the toggle, by default it is Ctrl + Alt + H)