I can confirm this UI bug.
The steps given to reproduce it are correct, with the exception of one thing:
this bug happens with the theme: Ambiance.
It may or may not be reproducible with other themes.
For completeness, I restate the steps here:
Preconditions:
Ambiance theme is currently selected
** Attachment added: "firefox-font-sizes.png"
http://launchpadlibrarian.net/50701397/firefox-font-sizes.png
** Attachment added: "Dependencies.txt"
http://launchpadlibrarian.net/50701164/Dependencies.txt
** Attachment added: "ExtensionSummary.txt"
http://launchpadlibrarian.net/50701165/
