Firefox Sync synchronizes your data across all of your computers and devices by storing it on Mozilla servers. To prevent others from accessing your account, you must specify an email address and password when setting up Sync. Steps to reset a Firefox Password:

• Click the More Tools icon in the toolbar, then select Firefox account from the drop-down menu.
• Click Forgot password on the Sign in page. Firefox will send you an email asking you to reset your password.
• To change your password, follow the instructions in your reset email.
• Remember to update your password on all of your synced devices after you've signed in with your new password.
• If the More tools button isn't visible, click the menu button, then Sign In next to Sync and save data.

Read More : How do I keep a Safari window open?