Intent to implement: DevTools onboarding page

# Summary When users try to open DevTools for the very first time, we would like to prompt them for a confirmation first, displayed in an onboarding page. The main goal is to inform new users about DevTools, and avoid accidental or malicious usage. A secondary goal is performance related. Some

Test helper ok() will now fail if used with more than 2 arguments

We are about to land https://bugzilla.mozilla.org/show_bug.cgi?id=1467712 which changes the behavior of `ok()` test helpers provided by Assert.jsm, SimpleTest.js and browser-test.js (as well as some test helpers based on them). `ok()` used to accept up to four arguments: condition, name,

Removing "clipboard" tag for mochitest

The `clipboard` tag was used as a workaround to run clipboard tests on a separate platform. This workaround was removed in https://bugzilla.mozilla.org/show_bug.cgi?id=1546459, making the tag useless for automation. I plan to remove all the `clipboard` tags from the codebase in