Are you talking about the Master Password?

https://support.mozilla.org/kb/Forgot+my+master+password Resetting your master password will remove all of your saved usernames and passwords. This is a security feature. Type chrome://pippki/content/resetpassword.xul<Enter> in the address bar.

Assuming that you are referring to a Firefox Account , used by the Sync service, your Saved Logins in the Firefox browser shouldn't have been deleted from the Firefox browser when changing your Firefox Account Password. But all your Sync data would be deleted from the Sync server with the Reset of the Firefox Account Password.

Are you using the Master Password feature? Did you lose any other data, like Bookmarks?

Have you connected to Sync with that Mobile device, with the new Password? If not - maybe Sync will pick up your Saved Logins from that device? i.e.; once that device is connected back to your Firefox Account / Sync. At least that is what is supposed to happen, but I don't have a Mobile device so I haven't used the Firefox for Android /iOS versions with the Sync service.