History showed 361 tabs had been closed, apparently by the install. One click and the tabs were back. So, fixed.

While this is a simple fix to a fearsome problem it shouldn't have happened. For diagnostic help, all I can tell you is that the previous version of Firefox was running during what I expected to be just a download. This morning I found that Firefox was closed/crashed, and when I restarted it the list of tabs was greatly truncated. This is the first time this (Firefox closed/crashed) has happened after a download in many years of using Firefox.