Having successfully installed (and updated) Linux Mint 19.1 MATE 64 bit on a spare laptop, I have come across a strange Menu Bar problem after opening Firefox (67.0.4).
As you can see from the attached pic-1 the Menu Bar is missing from the Linux Mint Start Page. This is normal (at the beginning).

I then press the Alt key to display the Menu Bar once (File, Edit, View etc) which works fine. So far so good.

I then click on View to bring up Toolbars (→ Menu Bar), Sidebar, Zoom etc).

The problem is that as soon as I move the cursor off View to select Toolbars, Sidebar, Zoom Menu, or any other option, the whole Menu Bar just disappears.

I am simply unable to select any options and I therefore cannot setup Firefox to my requirements.

This is the first time that I have come across this particular problem.

Any suggestions?

Attached Thumbnails

 

Try right-clicking into some small empty space. e.g. between URL bar and other elements, and put a ceckbox besides "Menu bar". Does that help?

ondoho,

Thanks.

I also tried that, with exactly the same result.

I decided to reinstall Linux Mint 19.1 MATE using exactly the same media and this time Firefox behaved as normal.
Very strange.

Maybe a reboot would have been sufficient?

Actually no because I had already tried that.