Paper menu not opening a second time in firefox

It seems that for ember versions >= 3.4 (I haven't tested in lower versions but it works fine in the demo page), `paper menu` fails to be opened more than one time in firefox. After the first time the button remains focused with the class `md-focused` and it doesn't open anymore. Removing that class with the devTools allows to open the menu again.

I've created a [repository](https://github.com/HenryVonfire/paper-menu-test) that reproduces this issue (`ember 3.5`, `ember-paper 1.0.0-beta.18`)

How to reproduce the bug:
1. open firefox
2. go to localhost:4200
3. click on one of the paper menu buttons (on the right)
4. click on the test option in the dropdown menu
5. try to click again on the same paper menu button (it won't open)