User Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/73.0.3683.75 Safari/537.36

Steps to reproduce:

CDP's Page.bringToFront is supposed to select tab and bring browser window in front of other applications in OS.

How do we do this in Gecko?

tabBrowser.selectedTab = newTab and ChromeWindow#focus() seems
to work for me when I test this in the Browser Toolbox.

This is incidentally the same technique we use in Marionette.

I wonder how hard it would be to implement this method!

Although unrelated to this bug, I have a question about how Puppeteer
deals with browsing context focus.

When “a page” is in the background, can you still interact with it
in Chrome? I.e. is it possible to click and type things into a
page living in an application window in the background?

We can reconstruct the CDP error and print a better error message
using RemoteAgentError. This is better than the current output,
which in a lot of cases will be "CDP Exception: [object Object]".

A possible future improvement to this involves extending the CDP
protocol to send the individual components of the error individually,
so that we don't have to parse the string.

Depends on D32538.
Not mandatory, but we have some duplicated code shared by several mochitests, maybe we could have a shared helper?

https://hg.mozilla.org/mozilla-central/rev/e2fbe8d063e0
https://hg.mozilla.org/mozilla-central/rev/4cf5a00f2fdc