You can run your own Sync server if you want [0]

[0] https://mozilla-services.readthedocs.io/en/latest/howtos/run...

I had ESR and regular Firefox installed so that I could use Google Hangouts last year after they dropped plugin support. After a particular upgrade (don't remember which), all of my data just disappeared because the profile formats were different and Firefox decided that the file was corrupted.

Thankfully I figured out what was going on, and they saved a backup of the "corrupted" file that I could just put back into place.

It seems like absolutely the correct solution to have each release to have its own profile.

Why would you assume they're trying to force you to use Sync? This change is being done at the request of a small subset of people involved in testing and development.

Is there a real world usecase where someone needs to have two versions of Firefox installed and it's beneficial to have shared profiles?

I would think someone paranoid about security and privacy would want different profiles so all your eggs wouldn't be in one basket.

Doesn't sync encrypt the data sent out with keys only known by you? So in that case the main loss in privacy seems to be that others learn that someone from your IP uses Firefox sync. Are there other privacy-oriented reasons to avoid it?

edit: Now that I think about it, it does also seem to force you to use a functioning email address as opposed to say an arbitrary username. That is something I never liked.

Firefox profiles are already not guaranteed to be compatible between versions. Sometimes this causes problems when sharing a profile between a Release version and an ESR version. For example, FF 55.0 moved favicons to their own file, favicons.sqlite, and FF 52 ESR would not be able to read favicons from the profile any more.

You can achieve the same effect by reusing the profile, though I've heard it can be buggy due to forwards/backwards compatibility issues esp. in add-ons. It requires a bit of technical effort (launching firefox with the -P flag to open the profile manager, or editing profiles.ini directly), but if you're using several versions of Firefox chances are this is not a problem.