How to install the extension manually?

[HubKing]

However, the extension can still be installed manually.

I tried to download the ZIP file and went to FireFox's Add-ons Manager, and chose install from file, but I got this:

If it can still be installed, could you add the instruction there?

[drunkwinter]

Yes, the readme is not very clear in that regard.

Firefox restricts you from installing any add-on that is not available on their add-ons website. You can technically install it via the about:debugging page, but it only remains installed until you restart Firefox. Practically, doing this won't make any sense unless you are a developer and want to test stuff. So it should probably read "cannot be installed manually". However I think there are still Firefox forks which do let you install unverified add-ons.