Are there downsides to using same profile from Fx29 to Fx54?

[c627627]

I have just become aware that there could be *major*, and not just 'cosmetic' problems if you use a profile touched by Firefox 55+ on Firefox 54 and earlier.
[Many thanks to Mark12547 for clarifying that to me.]

I believe I made my profile from scratch only two times. I think I had it from Firefox 0.x Beta to Firefox 28, and the last time I remade it from scratch, I think was in Firefox 29.
Thankfully, I kept backups so I can now go back to Fx54 Profile after I switch to Firefox ESR from Fx56 in order to avoid extension-ending Fx57.

1. Is it better to use my Fx52 Profile with Firefox ESR 52 or is using my Fx54 profile still 100% okay with Firefox ESR 52?

2. Are there downsides to using my profile since Firefox 29 but only with incremental versions up to Fx54? What are those downsides?
Thank you for your advice.

3. Can you "tell" which historical versions of Firefox 'touched' a profile folder just by looking at its contents?

If there are other issues, maybe someone else can post what they are, I would like to find out too.

To quote from Bug 1357428 - Profiles used with Firefox 55 will no longer work with older versions of the browser - Comment 6

Firefox 55 changed the on-disk format of persistent storage in profiles. Once a profile has been used with Firefox 55 (or later), it should not be used with previous versions of Firefox. IndexedDB, the (DOM) Cache API, Service Workers, and the asm.js cache will all fail to operate, generating confusing errors and causing portions of Firefox and some websites to break.

(Emphasis added.)

Part of what isn't enumerated here is the favicons, which is probably the more obvious but also probably the least important part.

Comment 6 of the bug continues with a proposed remedy:

The profile can be made operable again in older versions of Firefox by using the profile "refresh" mechanism.

[therube]

1. no idea
would think you'd be better off using the FF 52 Profile

2. if it's worked for you this long
you would think with each incremental update, FF would account for all necessary changes relating to the Profile & files within

3. no
you might be able to correlate certain files in your Profile with certain FF versions
like at some point, cache files went from /cache/ to /cache2/ & you might be able to say that ## version as for /cache/ & ##2 was for /cache2/, but without knowing or finding where changes happened, nothing in your Profile points to any particular FF version

[alexo]

Firefox 55 changed the on-disk format of persistent storage in profiles. Once a profile has been used with Firefox 55 (or later), it should not be used with previous versions of Firefox. IndexedDB, the (DOM) Cache API, Service Workers, and the asm.js cache will all fail to operate, generating confusing errors and causing portions of Firefox and some websites to break.

Does anyone know what files in the profile are used for the persistent storage mentioned?