qywter said

Also i just reset my firefox account login password,

Sorry, Sync data is deleted when the password is reset. Your data is no longer on the Sync server once the password is reset.

... i need to know the old password of that account as well in case both passwords being same which will save me from a lot of trouble. Tell me how to do this.

The password is not sent to the Sync server, it stays on the devices which are connected to the Sync server. Only the user ever has the original "password" that they created when they initially setup the Sync account / Firefox Account (as it is now called). And the password appears in the Logins Manager under the "Site" URI of https://accounts.firefox.com if they ever logged into the Sync portal. That feature is a recent enhancement to Weave/Sync which is like 10 years old now (Dec 2007); "recent" being within the last couple of years.

All "communication" between the individual devices and the Sync server are done via an algorithmically generated pairs of "keys" - kA & kB; one "key" is used to encrypt the data which is sent to the server and the other is used to log in to the server on each device. Those keys are able to be viewed in the Logins Manager > Saved Logins in desktop / laptop versions (I'm not sure how the Mobile versions display that information in Firefox, I don't have a Mobile device.) of Firefox under the URI of chrome://FirefoxAccounts (Firefox Accounts credentials) and look like this.

Username: 1a8b1109d03d44abd948ali91d571998
Password: 
{"version":1,"accountData":{"kA":"b8b02f83306f5f0e038c14b3de82f585b38a40fa1f30c75e75c9d95fcc4446e8",
"kB":"c7961085aab9766b95cae3cfe85319902fc991958cbdf33259276f645a9gk007"}} 

I havent yet reset my password and firefox account password on website and firefox sync password both are different. I need to know my old firefox account password not the sync password and need my prev synced data even if its in encrypted format.

Hi, sorry that does not happen as Mozilla does not retain the information as per the-edmeister. You are the only one with the password. Since you can not remember it you will have to reset as was explained. This is to do with security, yours. I would suggest that for legal purposes that you could ask your lawyer about a subpoena to prove that any recoverable information is actually yours. As well Support does not have access to anyone's info. G'luck with your legal issues.

  Thank You for Contacting Mozilla Firefox Volunteer Support.

qywter said

I havent yet reset my password and firefox account password on website and firefox sync password both are different. I need to know my old firefox account password not the sync password and need my prev synced data even if its in encrypted format.

Starting around the time Firefox 29 was released, Firefox Sync uses your Firefox Account password. So although you said your Firefox Account password and your Sync password are different, that is not possible.

The older Sync service was shut down, so even if you have a Recovery Key for that service, it's not of any use now. See: Firefox Sync upgrade - Frequently Asked Questions.

To make it a little more confusing, this support site uses a different password because it uses an ancient platform that hasn't been converted to Firefox Accounts yet.

yes i meant this support site password only, the one i used to login here is diff however their is a chance that my old password on this site might be same as my sync password so any chance i can get to know what my old password on this site was

Websites do not store your password, they store a hash of your password. When you try to log in, the same hash function is applied to the password you entered, and the two hashes are compared. The idea behind a hash function is that it is "one-way", so it is not possible to reverse the hashing process and tell you what your password is (assuming the hash function used was not a weak one). All of which is to say, no, if you do not know your password, you are out of luck.

anyway to get that hash of my old password, under the assumption that my password was weak one, better than nothing

Have your attorney contact Mozilla's legal department about your options. It's not a support issue.