There's not an easy way to override those files.

What behavior exactly are you trying to accomplish?

Many thanks Mike if you can help me.

Computers are on a local network, and sometimes we have to make some updates or sometimes a switch is down. The consequence is that Firefox show a message (in French, sorry) "Delai dépassé, ..."

I want to make a local page on each computers. The goal is to put this local page to replace the page generated by Firefox.

After, i can do that i want, exemple : I can put a WAMP server on each computers and offer a temporary service to offer an alternative.

I've searched in the omni.ja (aboutNetError.js), but don't really know if it's the good way. (Sorry for my English spoken and writen).

Edit : In aboutNetError.js, may be can i play with function initPage() to trig my local page and not the Default Firefox page error.

Something like :

function initPage() { window.location.href = "file:///C:/error/server_down.html"; }

Is it a good way ?

I know this is a very old question, but I never found a simple solution for you.

Modifying the omni.ja is probably the quickest way.

If you want to work together to come up with more complete solution, let me know.

It looks like you're doing a kiosk, and I might have some ideas.