Try this first:

orange Firefox button (or Tools menu) > Add-ons > Extensions category

Does the 365 extension have a Remove button? If so, click Remove and restart Firefox and you should be done.

If it has a Disable button but not a Remove button, you may need to uninstall through the Windows Control Panel.

Any luck?

More info: Disable or remove Add-ons

It had a remove button and worked perfectly.

An old codger thanks you from the bottom of his crusty old heart!!