I know it says the extension is not available from the Firefox addon site if using Russian IPs, but I wonder if they have also gone so far as to make the browser itself not be able to install them in other ways. I would guess they have not, since that would mean a complicated setup in terms of the signatures, like they would have to have a separate FF version and set of signatures per country, or use a central server to authenticate things rather than client validation of signatures. In that case it would be easier to find the addon file somewhere other than the store and install it, since using unsigned addons requires installing a whole separate version of Firefox.
Even if that's how it is this whole thing still illustrates that prohibiting unsigned addons from being installed is user-hostile, because on an ideological level Mozilla probably would use that power to advance state censorship if it came down to it.