Line break removed from paste in Firefox

[benbro]

[Describe the issue]
Line breaks are removed when pasting in Firefox.
Chrome works fine.

Steps for Reproduction

1. Visit http://beta.quilljs.com/playground/ with Firefox
2. Paste a text with new line

aaa
bbb

Expected behavior: [expected]
We should see 'aaa', new line, 'bbb'

Actual behavior: [actual]
We see 'aaabbb' on the same line.

Platforms:
Firefox 46.0.1 on Windows 7

Version: [version]
1.0-beta.4