FF4 combined a bunch of files into omni.jar -- unzip it to edit (it's a zip file, unzip to its own folder, don't just do it inside of the firefox folder). It's located different places depending on your OS; in Ubuntu, it's usually in /usr/local/firefox/ or /usr/share/firefox.

Depends on what you want to edit in the error page, but here are two possibilities inside omni.jar:

• omni.jar-unzipped/chrome/en-US/locale/en-US/global/netError.dtd
• omni.jar-unzipped/chrome/toolkit/global/netError.xhtml

You'll need to redo any changes any time you upgrade Firefox. Back everything up too, just in case!

Changes in Firefox 8 on Ubuntu 10.04 Lucid:

• omni.jar moved to /usr/lib/firefox-8.0/
• netError.xhtml is now in omni.jar-unzipped/chrome/toolkit/global/netError.xhtml

Changes in Firefox 9.01 on Ubuntu 10.04 Lucid:

• netError.xhtml is now in chrome/toolkit/content/global/netError.xhtml

And now in Firefox 11 on Ubuntu 10.04 Lucid:

• omni.jar is now called omni.ja
• netError.xhtml is still located in chrome/toolkit/content/global/