Editor looses focus in Firefox after typing two bold characters

[benbro]

[Describe the issue]
When typing two bold characters in firefox, the editor loose focus. The user need to click the editor again to keep typing.

Steps for Reproduction

1. Visit http://beta.quilljs.com/playground/
2. Click on the bold button
3. Type "aa"

The editor focus is lost. You can't keep typing.

Platforms: [ex. Chrome 48 on Mac 10.11]
Firefox 46.0.1 on Windows 7

Version: [version]
Beta 0