It works here in W7 and W10 with both the default and a different theme, so I wonder if your Windows theme, or possibly a non-default TB theme, is making it invisible.

Hmmm.... I use the Classic Windows Theme on my desktop. Perhaps it's that?

How would I know if I have a non-default TB theme? Would I have had to knowingly install that or might I have clicked an option at some point over the last ten years that flipped it on?

Look under Tools/Add-ons/Themes. If you're currently using the Default theme, try Dark or Light.

Interesting. I can see dividers in the toolbar if I am using the Light or Dark theme but not with the Default theme. I don't ever see an actual bar in the Right Click -> Customize area as the window background is always white. I do see "Separator" in text and I can drag one to the toolbar where it will show up per above. I tried to switch to/from the dark and light then back to default and default does remove them reliably.

Yes, you would assume that everything works with the Default theme, with any problems due to non-default styles. You can try one of the TB 60-compatible themes found here.

You mention you're using a Windows Classic Theme. My results in my first reply are with an Aero Theme in W7.

sfhowes: While looking over themes, I noticed this note " From version 61.0, Thunderbird will drop support for full custom themes."

Is that correct? That seems like I'll get my joy and then lose it just when it's getting to be fun.

Yes, just as Firefox has abandoned complete themes, the next release of TB, (perhaps 68, later this year) will probably only allow lightweight themes or personas. This is partly due to dependence on Firefox core code. Have fun while you can.

Ugh. I'm baffled how anyone with even a small clue about UI design could believe that the monochrome toolbars were an improvement. Was there an uproar when the usable ones disappeared?

Uproar or not, it wouldn't matter, as these decisions are not influenced by public opinion. Just look at the poor design of this forum.