See:

• https://support.mozilla.com/kb/what-are-tab-groups

Nothing on this page answered my question. I have closed all the groups and wish to return to the default so that when I load Firefox, my two default Home tabs load (which I chose because I want them) and not my last session tabs. I still do not know how to get rid of the tab groups completely and the above link, although it tells me what they are (I had already read it, so it was superflous), does not tell me how to remove them entirely. Each time I close Firefox now, when I reopen it, the last tabs I was looking at are displayed. I do not want to resume sessions every time. I want to start from my default Home pages. Anyone with ideas?

Reopening tabs from the previous session is not a feature from tab groups, but is the normal behavior of Firefox 4+ versions.

You can check the startup settings:

• Tools > Options > General > Startup: "When Firefox Starts": "Show my home page"

• Press F10 or tap the Alt key to bring up the "Menu Bar" temporarily if the Menu Bar is hidden.
• Use "Firefox > Options" instead of "Tools > Options" if the menu bar is hidden.

Thanks you cor-el. Simple and straightforward.