Sorry, I meant "Where are my TABS ?"

I'm not sure what happened. As a first step, could you check Firefox View to see whether your tabs are opened in a non-visible window? To open that, either:

• click the file drawer button at the left end of the tab bar
• type or paste about:firefoxview in the address bar and press Enter to load it

In the left column, click Open Tabs. Then in the main column, Firefox should show all the open windows. Are there any here that might have your missing tabs which are not otherwise visible on screen?

You might also reopen the last three recently closed windows in case one of those is useful. Either:

• "3-bar" menu button > History > Recently Closed Windows
• (menu bar or Alt+S) History > Recently Closed Windows
• Ctrl+Shift+N - reopens the last closed window

Anything so far?

Thank you so much for your kind help. Unfortunately, nothing worked. What shows in Firefoxview is exactly what shows on screen. And no, there's nothing in the Recently closed windows, just the last tabs I closed. ???...

You didn't mention any other changes, so this seems unlikely, but the first part of the following article describes how to check for other profiles: Recover lost or missing Bookmarks.

That was a good hint and I learnt something, but there is only the Default profile... Now, I am asking for help from the Simple Tab Groups community on Github... Many thanks again.