Using Firefox Nightly and switching between source / normal mode adds a newline to <pre> blocks

[SphinxK]

Steps to reproduce

1. Use ​http://nightly.ckeditor.com to type some text
2. Switch to source mode and add <pre>foo bar</pre
3. Switch back to visual mode
4. Switch again between source and visual mode

Expected result

No side effects from switching between modes

Actual result

A newline is added right after the opening <pre ...> tag, each switch adds another newline.

Other details (browser, OS, CKEditor version, installed plugins)

I reproduce this with Firefox Nightly only and on MDN (e.g. ​https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_objects/Map/set$edit ).

I'm sorry I cannot go further analyzing the issue with my knowledge.