I'd recommend making a backup of your profile folder on your desktop system (close Firefox before doing so, then just copy-paste it onto an external hard-drive).

But yes, resetting your password throws away your uploaded data from Mozilla's servers, which should be identical to the data you have synced locally, which it will re-upload after the password change.
So, no data loss should occur, unless you've simultaneously lost all devices that were synced + the password.

Are you just changing the password or have forgotten it and need to reset it? If it’s the latter I’m guessing you can’t reset without losing these things; basically you’re losing the encryption key.

The account does the stuff online, deleting it will not delete local data