Taking a look at some of the “experimental” add-ons for Firefox, this one caught my eye. ErrorZilla Plus looks like it is a refreshed version of the original ErrorZilla with a few more perks.
Here is a little more information from the developers…
This extension is a mod of 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. From Chinese users, it is especially useful because of the GFW’s cutoff. When hit a blocked website, just click “Proxify” and you will surf the forbidden area.
To use ErrorZilla you will need to register and login to the add-on’s specific page to gain access to it.