So, I did it

I took Firefox 57. NoSquintPlus (NSP) seems okay. Except there are no minimise/maximise/close buttons and the menu text, against a black background cannot be read.

I have tried various themes to change the background and they work only on the tab currently displayed at the time. It brings back the missing buttons. Note that I say brings back the missing buttons rather than simply having them visible because of a different background. Oh yes, the right click drop down menu for the same features doesn't work either.

Not only that, the moment I change tabs or open a new one, the theme I just activated disappears. It doesn't get saved. So I'm back to an unreadable menu and no min/max/close buttons.

Also, only the tab that is displayed has text I can read. The other tabs are just tabs with no clue as to what they are until you open them.

I went back to Firefox 56 but noted that NSP had updated (even though I'd set it not to update as later versions didn't work) but it still seemed to work.

I tried again, no change so went back to 56. But now NSP isn't working. So I shall have to re-install the version that was working before and maybe FF58 will work properly.

I realise FF is free and I'm so used to it that I don't want to change. But why can't they charge for their Browsers giving users the choice of what they want to pay for instead of having things foisted upon them?