FF Dev updated itself, created a new profile and somehow my copy of FF Prod ended up pointing at the dev profile. (i.e. all my bookmarks, plugins, history etc. "disappeared").
To tell FF which profile to use.
Close all instances of Firefox
Open the Terminal application.
In the Terminal application, enter the following:
Press Return. The Firefox Profile Manager (Choose User Profile) window should open.
Choose the correct profile.
Click Start Firefox.