Good idea, but it fails to function on Firefox error pages.
In the case that the website fails to load completely. (It works in the case when the webserver redirects you on an error page or elsewhere)

Since this is half the time that the addon would be handy it's a serious weakness.
Other similar WE addons work on error pages. For example:
https://addons.vaultpit.io/firefox/addon/google-cache/