Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. Is the problem still there?

Thank you, FredMcD. After entering Refresh Firefox, the amount of time allowed to name a New Folder tab in Bookmarks became entirely adequate. I followed your {web Link} https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode, but I lack knowing how to to sequentially select and progressively disable possible causes for the problem.

Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes). If the problem goes away, that tells us one thing. If it does not, that tells us something else.