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.