Hi When there is a major version update like from 91 ESR to 102 ESR then there is always an overlap of a few Firefox ESR versions to allow admins to test the new version and make sure that there aren't issues with deploying the new version. Firefox 91 ESR is almost at the end of its release cycle (Firefox 91.13 will be the last version) and 102 ESR is currently at its second release, 102.0.0esr and 102.1.0esr have been released so far. Once 102.3.0 gets released then this version will be the only supported ESR version (i.e. 91 ESR has reached EOL at that time).

Thank you, though I'm not quite sure if that answers my question.

I did some more digging and think I found the answer--please correct me if I'm wrong: The chart on https://support.mozilla.org/en-US/kb/firefox-esr-release-cycle suggests that there will be

1. an ESR update for the existing ESR version roughly 8 versions behind the current release AND 
2. a new ESR version for the current release itself

Both ESRs move forward in parallel, so existing ESR users continue to receive updates and progress ~8 versions behind while new ESR users can install the current ESR release, which will eventually lag ~8 versions behind the current release.

I think the fundamental question is: what changed between Firefox 91 and Firefox 102. A lot changed! I can't imagine anyone has a full list, but it would the cumulation of the changes noted in the release notes for Firefox 92-102.

https://www.mozilla.org/firefox/releases/

While I imagine there are a ton of changes between the two ESRs, an even more fundamental question is Why are there two ESRs to begin with?

While I imagine there are a ton of changes between the two ESRs, an even more fundamental question is Why are there two ESRs to begin with?

Most of the year, there is only one. However, we are in the annual transition period. ESR was invented for Enterprise users where the IT department needs time to validate and adjust to the new version. So there is about a 3 month period where the initial releases of the new ESR are available in parallel to the last 3 releases of the old ESR. After that, users of 91esr will automatically migrate to 102esr, and 91esr will no longer be supported.

Interesting.

That makes earlier comments make sense--I was under the impression that it was normal to have two ESR versions.

But it sounds like this is an unusual circumstance where there are two ESRs available.

I guess this is intended so the ESR doesn't fall too far behind the current release while giving enterprise admins 3 months to test the transition from the old ESR to the new ESR?

The ESR release is re-based on a current release once a year. It is intentionally "behind" by up to 15 months, accounting for the transition period. It does continue to get security patches during that time, of course. I'm sure there's more history/documentation from back in the days of Firefox 10 when it was launched.

ESR is meant for Enterprise users in mind as Mozilla never intended it for regular Firefox users in mind. Though some Linux distros may use the ESR versions for their provided package builds.

The only times Mozilla served ESR versions to regular users was for example Fx 78 ESR (78.15.0esr was last) served to Mac OS X 10.9, 10.10, 10.11 users https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr

Previously it was Fx 52 ESR (52.9.0esr was last) served to Windows XP and Vista users as Fx 52.0 and later Releases required Windows 7 or later as is still the case with releases now. https://support.mozilla.org/en-US/kb/end-support-windows-xp-and-vista

Thanks.

I am familiar with the role ESRs fill--I was just unclear as to how and why there were two ESR versions.

Am I correct in understanding that this 3 month two-ESR version overlap is intended so the ESR doesn't fall too far behind the current release while giving enterprise admins 3 months to test the transition from the old ESR to the new ESR?

Am I correct in understanding that this 3 month two-ESR version overlap is intended so the ESR doesn't fall too far behind the current release while giving enterprise admins 3 months to test the transition from the old ESR to the new ESR?

I don't understand the bolded phrase. There is a new ESR release every year. Why is there a new ESR release every year? I think it was because over time it becomes more and more burdensome to maintain divergent code bases, and this was a compromise to satisfy enterprise requirements within Mozilla's staffing constraints. (I'm sure some people would prefer that it not change for 2, 3, or 4 years or more.)

Thank you all for bearing with my questions.

I apologize for the lack of clarity. I actually think I understand how this works.

What I meant to convey is that as the ESR version and current release versions grow further and further apart over the course of a year, it sounds like there is a brief 3 month period annually where the current ESR version is transitioned to a new ESR version that matches the current release version. So, while there is really only one ESR track, for 3 months out of the year, there are two ESR versions which allows enterprise admins to test and transition their machines from the old ESR version to the new ESR version.

The next ESR version is a snapshot of the current release when this release reaches this next ESR point. While the release version continues to keep updates (next ESR is 102 and current release is already 103) then the ESR release only gets security fixes and fixes for possible regressions found in release that affect ESR as well.

Thanks everyone.

After re-reading the entire thread, I saw that @cor-el's original answer was sufficient. It just took some extended discussion for me to understand what was going on.

To any poor soul who happens upon this thread, I came across this article which affirmed that this is confusing and incidentally answered my question well after the fact

https://nakedsecurity.sophos.com/2022/08/26/firefox-104-is-out-no-critical-bugs-but-update-anyway/

ESR demystified

As we’ve explained before, Firefox Extended Support Release is aimed at conservative home users and at corporate sysadmins who prefer to delay feature updates and functionality changes, as long as they don’t miss out on security updates by doing so.

The ESR version numbers combine to tell you what feature set you have, plus how many security updates there have been since that version came out.

So, for ESR 102.2, we have 102+2 = 104 (the current leading-edge version).

Similarly, for ESR 91.13, we have 91+13 = 104, to make it clear that although version 91 is still back at the feature set from about a year ago, it’s up-to-the-moment as far as security patches are concerned.

The reason there are two ESRs at any time is to provide a substantial double-up period between versions, so you are never stuck with taking on new features just to get security fixes – there’s always an overlap during which you can keep using the old ESR while trying out the new ESR to get ready for the necessary switchover in the future.