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Stretch with MATE DE - odd new file association problem

Somehow I had messed up preferred application to open a file.
I fixed by selecting a file of appropriate type, selecting Properties, and setting preferred application.

That solved almost all my problems.

The only exception is when using Synaptic.
When I click on "Visit Homepage" it insists on using Firefox :<
It used to open properly with my preferred browser SeaMonkey.
All other HTML open correctly with SeaMonkey.

Is another place I need to set preferences?