about:neterror does not seem to be available. You can view all the available about pages by navigating to about:about in Firefox.

Firefox will open the about:neterror page automatically when there is a need for it (i.e. there is a network related problem).

Why do you want to open this error page?

• view-source:chrome://browser/content/aboutNetError.xhtml

Can I make changes to the page? the text, style, etc. How to make them effective?

That are built-in about pages that are stored in one of the omni.ja ZIP archives in the Firefox program folder. These files are accessed by Firefox via a chrome URI like I posted above.

See also:

• https://dxr.mozilla.org/mozilla-release/source/browser/base/content/aboutNetError.xhtml
• https://dxr.mozilla.org/mozilla-release/source/dom/locales/en-US/chrome/netError.dtd
• https://dxr.mozilla.org/mozilla-release/source/browser/themes/shared/aboutNetError.css

Thanks. But you did not answer my last question. Can I customize them?

I don't think that you can customize these pages. An extension might be able to overlay these the CSS file and a language pack can replace the entities as defined in the 'dtd' file, but you can't do this via built-in means.

You can try to use this selector in userContent.css

@-moz-document
 url-prefix(about:neterror),
 url(chrome://browser/content/aboutNetError.xhtml) {
}