There isn't any built-in feature to put the tab bar on a different row. The tabs normally slide up into the title bar when you maximize the window. Does that still work? If not, you could check to confirm that is still enabled:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste tabs and pause while the list is filtered

(3) If the browser.tabs.drawInTitlebar preference is bolded and user set to false, double-click it to restore the default value of true

If that's already true but the tabs do not slide up into the title bar when the window is maximized, hmm, are there any obvious impediments, such as the Title bar (page title) or Menu bar?

Thanks for the input, jscher2000. The browser.tabs.drawInTitlebar preference was 'true'. I removed the 'menu' bar when this started, to restore the operating space to the size it seemed to be beforehand. The Title Bar seems longer than I remember - I'm sure the open tabs used to be alongside, rather than above it? - Minimising, then maximising the window is having no effect.

I think the title bar (the full version of what appears truncated on the tab) might not leave any space for the tabs. How do you feel about turning the title bar off? There's a button for that at the bottom left of the Customize tab:

Customize Firefox controls, buttons and toolbars

Hmm, is there an Old Firefox Data folder on your desktop? If so, are there any recently updated semi-randomly-named folders inside? That would be characteristic of Firefox's Refresh feature, and perhaps the secret to your old configuration is stored inside. It's not always easy to tease out custom settings from an old profile folder but we might need to dive in there if we can't figure it out any other way.

Actually,it's not the 'title bar' (I'm not showing that) that has lengthened, but the one below the tabs, in which a web address would be typed. That's the one I want to shorten & slide the tabs into.

Note that you can also consider to switch to full screen mode and possibly have the tab bar and the Navigation Toolbar permanently visible (right-click: [] Hide Toolbars).

P.J.Rogers said

Actually,it's not the 'title bar' (I'm not showing that) that has lengthened, but the one below the tabs, in which a web address would be typed. That's the one I want to shorten & slide the tabs into.

I'm not aware of any built-in way to put the tabs into the main navigation toolbar. Maybe an add-on?

You would have to use an extension like Classic Theme Restorer to be able to move as much items as possible to the Tab bar and hide the Navigation Toolbar.