Check how FoxyProxy injects content to the Firefox error pages

In the “The proxy server is refusing connections” page FoxyProxy injects something like an ad: “Consider using fast and reliable proxy servers from FoxyProxy. (Hint: Please disable your current proxy if you get this error page while trying to load FoxyProxy’s proxy service website)”

Let’s see how it’s done, and if we can use the same method to add a useful message to the “Unable to connect” page shown when trying to access LAN in the Tor Browser. Bonus points if the message is only shown when trying to access the LAN instead of all the time.

Parent Task: #8711 (closed)

Original created by @anonym on 9466 (Redmine)