Earlier today when checking Tools | Extensions there was nothing
listed. Ditto for Themes. Both dialog boxes were empty. The
extensions were installed and active. The profile folder
contained all the extensions but the extensions.rdf file had
today's date and contained only the default list that comes with
a new install. Deleting the .rdf file and relaunching FF
recreated the .rdf file and all is well.

I also noticed that there was a backup extensions.rdf file called
extensions.001 dated 4/15 that appeared to contain all the
extensions and matched the restored .rdf file.

Any one have any idea what could have possibly caused this event?
In addition, how did the backup file extensions.001 come into
existence and is there a way to control this type of backup?