Allow start-url to specify a moz-extension URL

[andymckay]

Is this a feature request or a bug?

Feature request. It's a follow on from #647

What is the current behavior?

The URL can be domain or a page in the extension. However if its a page in the extension (such as foo.html) it is opened with file:///[path/on/the/file-system]/foo.html.

What is the expected or desired behavior?

If it opened at moz-extension://[UUID]/foo.html you could then execute the code in that. For example it could be a test suite. You can simulate this in the extension by adding something like:
browser.runtime.onInstalled.addListener(() => browser.tabs.create({"url": "/foo.html"}) );

But that's tiresome and means you have to change your add-on to do a build.

Version information (for bug reports)

v7.8.0
4.2.0
1.9.1

[aleclarson]

Alternatively, please allow a --start-url of /newtab.html to represent a URL relative to the extension. This way I don't have to determine the internal UUID of the extension just to define its start URL.