This is www-client/firefox-68.5.0. From time to time, when I switch desktops
to it, FF doesn't show the tab bar. The odd thing about it is that the mouse
pointer picks up displayed objects as though the tab bar were on display, so I
have to aim half an inch below what I want to select.
Then, pressing <ALT> shows the tab bar, not the menu bar, which remains
hidden, and the mouse picks up the right objects - until I click something,
when the tab bar disappears again. I think this is expected behaviour, given
Recompiling FF seems to be the only way to recover from this.