I was seeing this with the old docs pages as well, but no longer.
I was able to reproduce this issue. Not only on FF (both Linux and Windows) but also on Opera.
The problem is that those browsers fire a focusin event when you click on a linked button. This is not followed by a focusout event. In buttonMarkup we add the ui-focus class on focusin focus and remove this class on focusout blur.
The buttons don't need the focus styling because they already get the "down" and "active" styling. However, if we now prevent adding the ui-focus class, we might run into this issue again if we want to switch to using the :focus pseudo-class as part of #5173.
This should be fixed when we merge commit fb863df (branch "next") in master.
Tested on FF again after we merged branch "next" in master and could not reproduce it anymore. Closing as fixed.