Two clicks steps:
1) In the list of accounts, click Remove an account .
2) Click either the ‘Remove…’ button for the account you want to delete or highlight it and press Delete from your keyboard.
In order to delete your Firefox account, you need to log in. You’ll find the Delete Account button at the bottom of the screen. In addition, before deleting the account, please read and understand all of the effects on doing so. Once you do this successfully enter your Firefox Accounts password into this dialogue window and click Delete Account.
click the input field on the web page to open the drop-down list.
highlight a selection in the list and press delete to remove it.
To remove an account, sign out of the desired account and choose the signed out account from the dropdown menu. From there, make your choice to either Remove or Sign In.
If you delete your Firefox account, data stored in the Firefox browser on your computer will still exist. It will no longer sync with other devices.
To sign out of your Firefox Account on a device, click the Menu Button and then Sign Out.
To delete personal information synced with that installation from this device, click the Menu button and put a checkbox next to Delete data from this device.