Observe the large white areas on the URL. Make the browser full screen to see the bug. They do not appear in Chrome.
Please attach a screenshot.
Seems to work fine on my computer. Flags: needs-replication
Saw it on a 1024x768 screen with firefox expanded to fill the screen, and totally empty $HOME i.e., no profile.
With smaller resolution they both appear in Firefox and Chrome 29 (when increasing the font) for me. I don't see any valid bug in this bug report. With some font size and long lines without linebreaks this will always happen.