You are correct. With MacOS 10.10.x, you already have the latest version of Firefox that will run on your system.

Come April, Firefox 78 ESR will continue to browse as it does now (subject to more and more websites changing in ways that are not completely compatible).

What is changing is on the back end where you sign in to your Firefox/Mozilla Account. This change is going to break Sync in Firefox 78 ESR (and earlier). Possibly Pocket and other Mozilla services could be affected, too.

To protect against losing your Firefox data, you can keep Time Machine or other backups of your profile folder. See: Back up and restore information in Firefox profiles.

Thank you for the response. Could you possibly dumb down your answer another step? Specifically the last two paragraphs. What exactly does that mean to me? I am sorry to be such a Ludite!

On the third paragraph, I don't know the exact details of how the connection "handshake" is changing. You could think of it similarly to a new higher standard for secure connections that older browsers are unable to understand.

On the fourth paragraph, since all your data is stored locally on your computer (not in the Mozilla cloud), you may want to make backups in case your system dies or becomes corrupted.

I've noticed that on ye olde laptop running Firefox 78.15.0esr on Mac OS 10.9.5 that it does not Sync open tabs with either the 10.14.6 computer on 115.12.0esr or Firefox Mobile. But I can specifically send tabs to the laptop. (Its own "Send tab" option is grayed out.) Should I investigate further to see if the other checked sync items (logins, add-ons, prefs, etc.) are actually syncing, or is it expected that this connection is broken now also?

I've noticed that on ye olde laptop running Firefox 78.15.0esr on Mac OS 10.9.5 that it does not Sync open tabs with either the 10.14.6 computer on 115.12.0esr or Firefox Mobile. But I can specifically send tabs to the laptop. (Its own "Send tab" option is grayed out.) Should I investigate further to see if the other checked sync items (logins, add-ons, prefs, etc.) are actually syncing, or is it expected that this connection is broken now also?

Hmm, I thought nothing would sync (bookmarks, history, logins, open tabs) once the old authentication method was removed from the server. But it should be easy to tell by comparing your most recent bookmarks on the newer versions of Firefox (menu button > Bookmarks has the "Recent bookmarks" list in Firefox 89+; I can't remember where we looked before that).

Maybe the situation is: Fx78 can stay signed in and Sync will work in whole or in part until the next time you sign out. After that, your Fx78 will never be able to sign in again.