Add support for closing modals using the esc key as well as clicking the modal backdrop.

Thanks for filing, Emily! Is this something you're actively working on or just wanted to make sure there was a bug to capture this work?

(In reply to Jessilyn Davis from comment #1)

Thanks for filing, Emily! Is this something you're actively working on or just wanted to make sure there was a bug to capture this work?

Hi Jessilyn,

I've been working on the a11y linting for Activity-stream, so as I do that I've been filing bugs for accessibility improvements. For now it's just to make sure there's a bug to capture the work, though I may pick them up later if I can!
Bug 1553234, adding aria-expanded to collapsible sections, is another one.

https://hg.mozilla.org/mozilla-central/rev/07bb612a30b0

I have verified this issue with the latest Firefox Nightly (69.0a1 Build ID - 20190603160429) installed, on Windows 10 x64, Arch Linux and Mac 10.14.5. Now the modals can be closed using the "Esc" key.