Thanks., I had searched, but I obviously did not use the right words when searching.
I never had a thumbnail size slider visible until I added more thumbnail sizes. I asked a friend if he has a slider, and he does not.
What I mean by "I added more thumbnail sizes": See the screenshot in my original post. There are 6 different thumbnail sizes listed, as opposed to the usual 4. I added other sizes in Settings > Preferences > File Display Modes > Thumbnails.
Today I removed the thumbnail size button (the one with the pixel dimensions, not the slider) by dragging it back to the list of Commands in the Toolbar Editor, but now I can't find that button! It does not have an icon/image like other buttons do! How can I find it to put it back on the toolbar?
I can't see an option for that, but I may just be overlooking it and it's not too important so don't worry about it. (FWIW, I think the number of sizes can be changed by editing the command that generates the size list.)