Hi Taxlady, you should still be able to rearrange tabs by dragging, and also to "tear off"/"detach" tabs to their own window.

There is a recently added preference related to detaching tabs that could disable that second function. You can check here:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste detach and pause while the list is filtered

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

More info on about:config: Configuration Editor for Firefox.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Thank you both for your replies. I had a stressful week, or I would have gotten back to this sooner. It may have been a Windows problem. It spontaneously went away when Win 10 did an update.