Firefox does allow multi-selection of tabs, and in that case Duplicate Tab will change to Duplicate Tabs (in other words, duplicate all the selected tabs). Is that happening somehow?

You mentioned grouping tabs by subject. Is this just a matter of sliding them near each other on the tab bar, or is an add-on involved?

Thanks for Reply. I do select multiple tabs to force refresh of a group of tabs. When Problem occurs I am selecting one tab and invoking Duplicate Tab. I am manually sliding tabs in tab bar for grouping. Add-ons are Adblock Plus and Privacy Badger. I do NOT use Sync between devices. Currently about 100 tabs are open. I will attempt to take a photo of screen tabs when problem occurs but that will require a restart of Firefox to recover. The tab I am trying to duplicate is about 70% through tab list. I suspect (not confirmed) that all preceding tabs are being copied as some of the tabs I recognize are in first group of tabs regarding Weather info.

Can you duplicate a tab otherwise like with middle-clicking the combined Stop/Reload button or holding the Ctrl key and drag the tab (i.e. does it only fail with the context menu) ?

Unexpected result. Middle-clicking the combined Stop/Reload button works OK. Before proceeding further I got notification of Firefox Update. I checked Help/About Firefox and the install showed as 97.0 and not the 97.0.1 which was shown before I made first posting 2/18/22. I proceeded with Firefox Upgrade to 97.0.1 and restarted Firefox. Duplicate Tab is now working OK. I have no idea why version in Help/About Firefox changed or why an update to the version I started with appeared to fix problem. I can only assume the Update Client found version (not current) and did reinstall. This Tab problem has occurred in past few Updates so I guess it might be a procedural problem. Firefox is often restarted but PC restarts are rare. Problem resolved before I could determine if it only fail with the context menu. Kind Regards...