Not sure about January 14 however the message was likely just stating any Extensions you have installed will not work starting March 14, 2025 unless you have Firefox 128.0 or Firefox 115.13.0esr or newer (currently at 115.18.0esr).

Important update! On March 14, 2025, a critical root certificate in Firefox will expire. If you’re still using an older version (before Firefox 128 or before Firefox 115.13 ESR), it’s crucial to update to Firefox 128 or 115.13.0esr newer to avoid issues with add-ons, DRM-protected content, and other features. For full details, check out Update Firefox to prevent add-ons issues from root certificate expiration.

*Will my current version still work after January (but with no new security updates)?

• or will I not be able to use my Firefox browser at all?

Yes however there have not been any security updates for the old Firefox 78 ESR since the last one Fx 78.15.0esr released back on October 5, 2021.

Sounds like you can still use the old Firefox 78.15.0esr (based on the Fx 78.0 Released June 30, 2020), just that you may likely not be able to use any Extensions, and likely not currently supported by DRM content anyways. There will be an increasing number of websites that may not work properly but does work in more current versions and of course you can be vulnerable to known security issues that are fixed in more current Firefox versions.

I am not computer literate so I cannot update my Operating System so I won't be able to download a more recent version of Firefox browser.

You probably can run a newer version of MacOS, but this is not considered a regular update to Mavericks, so the update checkers might not inform you. Apple calls is an upgrade and I suspect they might charge for it.

According to an old Apple support thread, you can go to the version's page on the store and there is a compatibility checker feature.

• Monterey (12): https://apps.apple.com/us/app/macos-monterey/id1576738294?mt=12
• Big Sur (11): https://apps.apple.com/us/app/macos-big-sur/id1526878132?mt=12
• Catalina (10.15): https://apps.apple.com/us/app/macos-catalina/id1466841314?mt=12
• Apple support article: How to download and install macOS

I saw a suggestion to try it in Safari if Firefox and Chrome don't allow the site to fully determine compatibility.

As a Windows person, this is all second-hand.

Thanks James and jscher. Last night I read some posts on this support website (I can't remember which section) and other people are just like me, we can't update. They want Mozilla engineers to continue supporting us.