How to override browser page content with WebExtensions

Add-ons Add-on Support
1

It is possible to override default firefox pages with chrome.manifest.

For example:
override chrome://global/content/netError.xhtml chrome://offline/content/netError.xhtml

How can I do this with package.json or with Javascript API for WebExtensions addon?

2

For WebExtensions (ones that have manifest.json), it is not expected that this is directly possible. Limited wrapped APIs for modifying more parts of Firefox could be added, I think.

For SDK-based add-ons (with package.json), you can use chrome.manifest. It may not be recommended, however.

3

Is there no possibility to access this webpage netError.xhtml page?

For example I did this and run jpm run command;

pageWorker = require(“sdk/page-worker”).Page({
contentScript: “console.log(document.body.innerHTML);”,
contentURL: “chrome://global/content/netError.xhtml”
});

And the netError page content is printed in console.
PageWorker is in regular intervals printing this value and I am not able to modify the content.