Keep an eye on Shadow DOM support in Firefox

[sbabcoc]

To get the level of support I've been able to manage, I had to create a special FirefoxShadowRoot search context. This works up to a point, but falls over when attempting to interact with input elements. There's a bug in the Mozilla issue tracker that was logged four years ago: https://bugzilla.mozilla.org/show_bug.cgi?id=1503860

Keep an eye on this and be ready to roll out corresponding revisions if/when this issue gets fixed.