See if this helps; Check your zoom controls.

<Control> (Mac=<Command>) + (plus) enlarge - (minus) reduce 0 (zero) restore

No that didnt work. It looks like a bug, but I found a workaround. As long as Firefox is maximized before I go into "clear history", the buttons are shown. If I maximize firefox after I go into "clear History" the buttons arent shown.

Thanks.

Zooming out (Ctrl + [-] key) the about:preferences page before opening this opp-up window should also work unless the window size is very small.