Phew! You're right! That sorted it all.
That's a fine add-on, is "Classic Theme Restorer." I think it's the only one needed. It put the missing stuff back, allowed the tabs to be the bottom toolbar rather than the top one, stopped them greying out, stopped the screen jumping at each tab change, and stopped the main screen "centring" when I try to snip a piece of whatever's on display (hence moving what I want to snip right off the screen in some cases).
The only problem left is that the font for the toolbars is tiny. I'll have a look for that setting next.