Paste button disabled when config.enterMode is used

I'm using ckeditor version 3.6.4. The only change in config.js is adding
config.enterMode = CKEDITOR.ENTER_BR

Only in Firefox (15.0.1), whenever I press enter, the paste buttons suddenly becomes disabled. Ctrl-V still works, and once I do anything else, they will become enabled again.

Any idea what's happening here?