A download of an app to the phone, initiated from the laptop using Google Chrome, works properly.
I made another backup and installed the FF 90.0.2 update, and the same lack of tabs occurs, so in the Chrome browser I follow
https://support.mozilla.org/en-US/kb/recover-user-data-missing-after-firefox-update
There was one other profile available, so I followed directions and it just opened another 90.0.2 window without saved passwords, or theme, or bookmarks, but with the tabs above the address field; previously I had the tabs below, so maybe that's the problem with the missing tabs.
FF 90.0.2 still exhibits the app-download problem that started this thread, so if I could get it to display tabs then I'd probably keep it, and just download apps from the phone as suggested.
At any rate, the FF 90.0.2 update doesn't work right ... it either preserves my passwords, bookmarks, add-ons etc. but with no tabs visible, or it displays tabs but removes all that's important to me.
<edit>tabs later restored with code from the FF forum. Original problem in this thread persists with FF 90.0.2</edit>
Mozilla really screwed up this one.