HTerr Comments

 That depends on whether the response from the server is an actual error message, or if the page is just visually blank. If the server returns an HTTP actual error code, the replacement is instant. If there's no error code from the server, the page will be "visually" checked (essentially a handful of pixels' colors are compared to each other) after (by default) ten seconds. This can be modified with the "extensions.hterr.blankwait" preference in about:config. Note that this scan does not take place until the base file of the page you're viewing has been completely loaded.

 It's entirely different; doesn't use the built in netError system, only its CSS.
 This extension literally just inserts body data into a 4xx or 5xx HTTP response using the 'http-on-examine-response' event of the Observer service. That means the error page you get from this extension is just a regular webpage according to the browser, and even the MIME type can't be altered, so if the empty HTTP error has a content-type of text/plain for whatever reason, the error has to be plain text instead of a webpage too, or it'll just show you the page's source code.
 It's more of a stop-gap measure for sites that send HTTP errors but no accompanying HTML data, so they don't display a blank page, rather than a true error handler.