Tested on Mac OS X 10.14 with the try build provided by Dave: https://treeherder.mozilla.org/#/jobs?repo=try&revision=f96dd8243e9203ce63e69c957da437377426ef3f
[Steps to reproduce]:
Open the browser
Press "CMD" + "Shift" + "P"
Also, you can do some actions from the Mac Menu Bar
The new window command is working, the Mac Menu bar can be accessed
When the modal window is displayed no actions should be allowed until you dismiss it.
Here is a screen recording with the issue: https://streamable.com/rgvh7
We tested on Windows 10 and we didn't reproduce this issue.