This extension is originally based on the ErrorZilla Mod extension. I added a "Proxify" function into it. When error page shows, you can choose a web proxy from a list and browse the unreachable network address via it.
Notice that if you install EZP with other extensions (e.g. Mr Uptime, TryAgain) which also replaces the official error page, only the last one will show.
If you find the proxification does not work, there is possibility that the problem is related to the web proxy. For example, some proxies may store temporary cookie at the first time you enter their web sites in a browser session, therefore you may fail to proxify at the first trial (it would bring you to their web sites, not the destination) but succeed afterwards.
A helpful free web proxy list can be found here: http://proxy.org/cgi_proxies.shtml.
Generally, this add-on does not need any update for future versions of Firefox. You can refer to this page to bypass Firefox extension compatibility check.
Github repository: https://github.com/CrendKing/errorzilla-plus