Sync doesn't actually "synchronize" extensions, it installs extensions afresh from the AMO website automatically. The only user-set preferences or saved data for those extensions that might get synchronized was saved by each each extension in the prefs.js file; totally dependent upon how an extension was written or configured by each individual extension. The more complicated the extension is, the greater chance that extension created its own data or preferences that don't get saved to the prefs.js file.

Some extensions have preferences hidden in about:config - some of which were created by that developer to enable Sync features for their extension. IMO, there is a major problem with how the user is (usually not) made aware of those hidden preferences; and while most extensions do not properly [IMO] document those hidden prefs, the few that do provide that information don't conform to any "standard" for providing that information. And that happens due to no "standard" for displaying that information was ever created by the Addons.Mozilla.Org website developers / maintainers.

IMO, Sync is useless when it comes to "migrating" a Profile to a new computer if the user is using more than a few, simple extensions or extensions that require much in the way of user settings or user-set preferences.

"... so how do i move my past history/databases over so that my migration is complete/seamless?" That would have to be done file-by-file and/or folder-by-folder for each extension. And very few extension developers provide information about exactly what their extension adds to the Profile folder. So basically the user is left to poke around the Profile folder and recognize which files / folders aren't part of the default Profile and then figure out which extension each of those "added" files / folders "belong to". Then manually copy over those files & folder into the new Profile on the new PC. " i tried moving the whole contents of profile folders in both local+roaming before synching, was i supposed to do that after login/synch?"

Only the Roaming Profile" folder contains the data that you would need. "Local" contains the cache and files that are of a temporary nature; not any data or preferences that would bee needed by an extension.

"... but would like to get an url to a bulletproof migration instructions for those who rely on tab mgr extensions and histories."

Sorry, I know of no complete set of migration instructions. And I have no experience with any "tab manager & histories" extensions. Look to a support website for the specific extension that you are using; that information can be found on the download page for that extension, on the right side of the page as Support Site if that extension developer provides that level of support.

The "drill" is, once Firefox is installed on the new PC and has been launched once, copy the contents of the existing Profile into the new Profile folder - replacing any existing files (yes all the newly created files in new Profile should be overwritten when Windows asks as part of the "Paste" feature in Windows.)

That is what I did when I "saw first hand" how poor the Addons Sync feature actually works when I was setting up a new PC in May 2015. I wasted a week or so trying to "fix" what got lost in Addon Sync, before I finally gave up and just copied everything over to another "new" Profile that I created. Too much time wasted on a half-@ssed feature that was poorly implemented and very incomplete documentation that was provided by Mozilla and the various extension developers.

so it sounds like closing firefox and re-copying the appropriate folders over the existing ones might yet solve/complete the migration. is that right?

and is it the Roaming\Profiles folder or the actual customized folder inside 'Profiles' that i need to copy over? that's where i'm not clear yet.

Type about:support in the address bar and press enter.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

This is your current profile. Copy the folder As Is to a folder on a thumb drive. Then Copy the folder to the new system.

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

• bookmarks and history: places.sqlite
• bookmark backups: JSON backups in the bookmarkbackups folder
• SQLite files like cookies.sqlite (cookies) and formhistory.sqlite (saved form data)
• logins.json and key3.db (decryption key) for Passwords saved in the Password Manager
• cert8.db and possibly cert_override.txt for (intermediate) certificates stored in the Certificate Manager
• persdict.dat for words you added to the spelling checker dictionary
• permissions.sqlite for Permissions and possibly content-prefs.sqlite for other Site Preferences
• sessionstore.js for open tabs and pinned tabs and tab groups

You can use this button to go to the current Firefox profile folder:

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

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