Greetings
I'm not sure many are in a similar position than I am since I haven't allowed automatic updates for Firefox ever since they disabled pure ALSA functionality. Nevertheless it is possible that others will behave as lazily as I recently have by not reading the Changelogs.

Recent versions of Firefox are incompatible with previous versions of Profiles. If you are not very careful you can lose all your Bookmarks, History, Passwords, etc. I don't wish to belabor the point, just make people aware of another fundamental change so that you read up first and take measures to secure your hard-earned work.

Good Luck.

I have had no issues with Firefox updates on any platform.
I have been sure to stay current...so have not suppressed automatic updates.

That was long ago.
You haven't updated since then? So why did you now?

In any case I have stopped complaining about this long ago; I wrap firefox in apulse instead:

The following my be of use:

FWIW I didn't say I never updated Firefox. I said I disallowed automatic updates. Since Slackware -Current provides the option to enable a Pure ALSA system, I have updated with versions provided by Slackware from time to time and experimented for a time with Firefox-fuckPA.

Yes I know how apulse works. This is not about that. This is about breaking compatibility with Profiles. You may find yourself unable to keep what may amount to a lot of hard work and lose considerable accumulated data. If you don't, cool. This is just a heads up. Take it for however you wish.

Thank you sevendogsbsd. I know most of that but am only now seeing that json is a key factor. I know almost nothing of json and what it does and how it works. How does one avoid losing data if and when json is updated?

json is just a data format - the files I referenced are in your Firefox profile. I have had this happen for different reasons so just keep notes on which files to restore in the even there is a problem. I don't think any of your existing Firefox data is mangled or changed; I think Firefox just creates a new profile if it deems this necessary.

I had to switch to Chrome because Firefox doesn't handle vsync properly, either on Linux or on Windows, and was causing YouTube videos to screen-tear unless I activated NVidia driver options that increased my input latency (specifically ForceCompositionPipeline).

You can do your own vsync tests here:

https://www.vsynctester.com/

This does not seem right. Immediately upon launching Mozilla-Firefox-68.6.0ESR I was greeted with a Firefox "What's New" page that informed me that my previous profiles were no longer compatible requiring me to create a Firefox account including "Sync" to recover any lost settings and data from previous profiles.

FWIW I just had to bring up a new laptop for my better half. I simply rsync'd from her old (Mint 19.1) to latest 19.3 with F/F 74.0 (??? - I think from memory). Told it to use default profile, and bingo - one happy household.
Same for T/Bird if anyone is interested.

Couldn't be easier.

Great to know syg00 and I realize I'm making it harder on myself by despising Pulseaudio (I need lowest latency for DAW work) but I must use ESR versions of Firefox in order to maintain compatibility with my desired sound server, ALSA. I know Pulse makes things simple and convenient for casual users but I have literally thousands of dollars invested in my sound system and I won't have it laid low by some one-size-fits-all low performance software. So it's worth the extra effort even though Mozilla has unnecessarily made it a lot harder.

UPDATE and Success-

After creating a bit of havoc on my old Slackware 14.2 system, in which I had upgraded json, I installed a new instance of -Current on another partition and copied the /home/.mozilla folder from 14.2 to -Current. It still told me i needed to create an account to launch Sync but I ignored it and went right to "aboutrofiles" and selected my previous hard-earned profile. That worked a treat without a hitch.

I think the havoc came from the json upgrade rather than directly as a result of Mozilla. Still I am displeased with Mozilla's manipulative arbitrary changes, especially since right before the ALSA fiasco (it's still in there, pulse can't operate without it or something like it) I actually donated to Mozilla...lesson learned.

The profile.default is now being changed into profile.default-default? This is just trolling, I see no other reason for renaming this directory.
Also, there are now 2 profile configuration files to look into, because one file apparently can't do 2 things at the same time.
Another thing I've seen, it failed to start because it's 'already running' even though the PID does not exist, it decides that it's running by detecting some random file.
(I don't use it for a long time, just checked it out)

I wasn’t impressed with that Microsoft-grade user astonishment, but you’re wrong [ EDIT: were wrong, I see you corrected that in a later post ] about FF account and Sync - it’s an option and prominently promoted, but not required.

I opened my backup browser (Chromium based, which has also caused unpleasant user astonishment; all browsers have disappointed me at some point) to see what Mozilla had to say about it.

It turned out to be easy, although it took some time: I found my old profile and new profile directories, made backups of each, copied all files and folders from my old to my new, et voilà, Firefox-68.6.0ESR looks and acts like Firefox-68.5.0ESR.

You could do it more surgically and copy only the specific files and directories you need (Mozilla’s website has a guide), but my brute force approach worked for me.

Someone who doesn’t know how to or want to dig a bit might just give up and switch to Chrome. Mozilla should do better about warning and guiding through this kind of change.

TKS

An even easier way. I tried that first, before copying all profile files and folders from old to new and switching back to my “new” old profile. Both ways work.

TKS