I have reproduced the problem (with FF45.5.1) by going to the menu toolbar and selecting "View->Zoom->Zoom Text Only" and then zooming in. NB: this does not happen (for me at least) if the "zoom text only" option is off.
There's an alternative way to change this setting (at least on my older version of FF):-
- Type "about:config" in the address bar
- In the search box type "browser.zoom.full"
- Set it to "true"
Crtl + and Crtl - works in most browsers.
In case it is a zoom problem, the OP could try pressing Ctrl-0 (zero) to reset the zoom.
It might, I should have time to look at this tomorrow.Looking closely at the overwritten text, it seems to be a mixture between the text that should appear when opt+ is invoked on a parent directory
and the text that should appear when opt+ is applied to an individual file. (and vice versa) I don't know if this gives a clue to the problem.