This question from 2021 talks about problems with the Profile.ini encountered the last time I upgraded Firefox: Profile I desire is not allowed to be used with "old" FF version https://support.mozilla.org/en-US/questions/1356226

And the addon Theme and Font Size Changer is essential for me, otherwise the screen display is usually too miniscule for me to read. Has it been fixed to run on newer revs, or is there an equivalent, or a built-in functionality?

from 2020: https://support.mozilla.org/en-US/questions/1317977 How to upgrade but make reversion to previous easy?

It might be that Macintosh Firefox 78.15.0 esr (64 bit) and Thunderbird 60.9.1 are the newest revs that work with OSX 10.11.6 El Capitan.

Correct: Firefox Mac OS X 10.9, 10.10 and 10.11 users move to Extended Support Release.

OTOH I don't know if the newer OSX will run on my 2011 MacBook Pro. If I clicked on "software update" in About This Mac, what would happen? would it just start doing it, or tell me about it first?

This would be better directed to a Mac forum.

There used to be a Mozilla web page with all the versions & you could download the dmg file you wanted.

This still exists, but you have the latest compatible version. Later versions *should* refuse to install.

In case of need: Install an older version of Firefox.

Firefox 116.0 and later requires macOS 10.15 (ten.fifteen) or later to run.

https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support

It seems that there are two later OSX that will run on my 2011 MBP, High Sierra and... the one after ? Monteray ??? Looks like its time to buy a newer computer to run an OS only a few years old, if not very new. Then I can pick newer Firefox and Thunderbird versions.

I just upgraded the wife's Firefox to 72.0.2 (64 bit) by using the ABOUT popup to upgrade. Now, the ABOUT box offers to upgrade to 109.0.1 . However, I read elsewhere that: How To Upgrade page URL : "If you are on version 70 or below Due to changes with how the address book is stored, you will have to manually upgrade to version 78.14.0 first. This will upgrade your address book to the newer format that later Thunderbird versions use. "

So I am not sure whether v72 already has the new address book format or not. What was the first version using it? I don't know if I can switch to any of the ESR releases. Afraid to try in case I ruin address book / profiles. I need to dig out the help pages on manually updating, I recall it was difficult, having to find hidden .ini files, and go into one with a text editor and change the pointer line...

MY computer is running 78.15.0 esr, and offers to upgrade to 102.15.1 esr.

I'd like to get them both on the same esr version of Firefox. Both are running OSX High Sierra 10.13.6 now. Maybe I should upgrade the wife's to

https://support.mozilla.org/en-US/kb/switch-to-firefox-extended-support-release-esr with the page customized to Ver 78, leads me to:

Switch to Firefox Extended Support Release (ESR) for personal use https://support.mozilla.org/en-US/kb/switch-to-firefox-extended-support-release-esr

which says the current esr is Firefox ESR 115.8.0 . I cannot customize the page to anything less than ver 78, so I don't know the esr to choose to use from regular ver 72.

Are you upgrading Thunderbird, Firefox, or both?

Firefox is not affected by the address book format in Thunderbird. You can ignore Thunderbird articles for purposes of Firefox.

If you need advice on Thunderbird, it's a good idea to start a new question on that forum:

https://support.mozilla.org/questions/new/thunderbird/form

Upgrading two computers, the OSX, Firefox, and hopefully Thunderbird but I don't want to mess with "manual" installation.

So the "If you are on version 70 or below" quote was from a Thunderbird page? Gets confusing when the revision numbers are all similar. However both computers are using a TB under 70 so it seems that will be a lot of work.

For Firefox, I guess I can accept the upgrade to the newer esr offered in ABOUT.

But what's the best strategy for the wife's computer, where I want to change from the regular release to an esr release? If I pick an esr release higher than the regular release it is using, will that transfer my Profile & bookmarks, etc ?

THIS: - the wife is on 72.0.2 . https://support.mozilla.org/en-US/kb/switch-to-firefox-extended-support-release-esr > Firefox now includes downgrade protection to prevent corruption of user profile data. Starting with Firefox 68 ESR, you'll be prompted to create a new Firefox profile if you previously used a HIGHER version of Firefox (read this to learn more). >

Does this mean that if I install a version of esr higher than regular 72.0.2 , I will NOT get a new profile?

The grammar is ambiguous - does it mean "if you previously used a HIGHER REGULAR version THAN esr 68" OR "if you install a version of esr higher than your current regular version, you will NOT get a new Profile?"

THIS: - the wife is on 72.0.2 . https://support.mozilla.org/en-US/kb/switch-to-firefox-extended-support-release-esr > Firefox now includes downgrade protection to prevent corruption of user profile data. Starting with Firefox 68 ESR, you'll be prompted to create a new Firefox profile if you previously used a HIGHER version of Firefox (read this to learn more). > Does this mean that if I install a version of esr higher than regular 72.0.2 , I will NOT get a new profile? The grammar is ambiguous - does it mean "if you previously used a HIGHER REGULAR version THAN esr 68" OR "if you install a version of esr higher than your current regular version, you will NOT get a new Profile?"

A profile is marked with the last version used for that profile.

If you install a later version, Firefox upgrades the existing profile. This is what happens during routine updates.

If you install an earlier version, you will be prompted to create a new profile because Firefox cannot downgrade the data files in your profile to work with a previous version.

Now... if the ESR creates its own profile instead of taking over the standard release profile, you will need to change profiles to use the one from the standard release. Check the steps in the first section of the following article on how to switch profiles: Recover lost or missing Bookmarks.

Does SYNCH provide a way without manually transferring a Profile? 1. create an account and let SYNCH grab the data. 2. Install or update a newer version of Firefox, which might ceate a new Profile or not. 3. If it did create a new profile, use SYNCH to add my old Profile. 4. Delete the "new" profile.

https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer

Maybe Sync would work for you, but I do not consider it reliable enough, or complete enough, to substitute for the procedure I mentioned, which is *very* simple:

(1) Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

This page should list at least two profiles. Do not use any Remove buttons!

The new profile that Firefox is currently using will have this:

This is the profile in use and it cannot be deleted.

(2) Is there a "default-release" profile listed on the page? Check whether it is the one you were using recently by clicking its Launch profile in new browser button.

If it isn't what you want, simply close that new window.

If it IS what you want, back on the about:profiles page, click the Set as default profile button for that profile so Firefox uses it automatically at the next startup.

(3) If not #2, try with a profile named "default" (or if there is a profile with a bunch of numbers at the end of its name, try that one).