Sorry, I have no idea. Sometimes it ports over just fine and in others it is corrupted. I do not run Sync myself. As for Sync I would guess would run the same risks. I have little info on it.

• https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer
• https://support.mozilla.org/en-US/products/firefox/sync
• https://support.mozilla.org/en-US/kb/how-do-i-choose-what-types-information-sync-firefox
• https://support.mozilla.org/en-US/kb/firefox-sync-troubleshooting-and-tips
• https://support.mozilla.org/en-US/kb/change-primary-email-address-firefox-accounts

As for loosing stuff all add-ons will be disabled after June as EOL for ESR and new will be Quantum based. Will have to look for new add-ons/Extensions https://addons.mozilla.org/en-US/firefox/extensions/

Firefox 52.9.0esr out on June 26 (same time as 60.1.0esr) will be the last major update for Firefox 52 ESR apart from any 52.9.x esr updates for security or allowed stability fixes.

Firefox 60.2.0esr will be out Aug 21 and Firefox 52 ESR will be EOL then. So people will have to upgrade to 60.2.0esr to keep getting security updates if they have not manually upgraded to 60.x.o ESR already. Firefox 60 ESR will require Windows 7, 8, 10.

Can just try but do so with copies of the profile :

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data
• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

Transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over a problem. Never work with original files.

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Please let us know if this solved your issue or if need further assistance.

You normally only need a small set of data to recover if you need to downgrade from Firefox 59 to 52 ESR.

You would already have to make sure not to launch Firefox immediately, but launch the Profile Manager to create a new profile. Once that new profile is created then you can recover some data. Since you had used 52 ESR with the old profile then you should still have key3.db and logins.json to recover the passwords. You can't use places.sqlite, so you would have to restore bookmarks from a JSON backup within the bookmarkbackups folder. If you copy the full folder to the newly created profile for the 52 ESR version and delete all places.sqlite files including possible places.sqlite-shm and places.sqlite-wal files then Firefox should recover the bookmarks automatically when a new places.sqlite is created.

You can copy files like these with Firefox closed to the current profile folder to recover specific data.

• bookmark backups: compressed .jsonlz4 JSON backups in the bookmarkbackups folder
• cookies.sqlite for the Cookies (only if you really need this, don't know if there will be issues)
• formhistory.sqlite for saved autocomplete Form Data (only if you really need this, don't know if there will be issues)
• logins.json (passwords) and key3.db (57 and older) (decryption key) for Passwords saved in the Password Manager
  if you only have key3.db then make sure to remove an existing key4.db
• cert8.db (57 and older) for (intermediate) certificates stored in the Certificate Manager
  if you only have cert8.db then make sure to remove an existing cert9.db
• persdict.dat for words you added to the spelling checker dictionary
• permissions.sqlite for Permissions and possibly content-prefs.sqlite for other website specific data (Site Preferences)
• sessionstore.jsonlz4 for open tabs and pinned tabs (see also the sessionstore-backups folder)

See also:

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile