OK, fixed it. The associations were not in Win7, but FF8:
In FF8: Options/Options:
for "csv File", changed "Action" to open with Libre Office Calc".
Now it downloads the .csv files and opens them in Libre Office Calc.
I Googled phrases from the error message and found a solution in an old Mozilla page.'
Google is your friend, but you need to ask the question correctly.
Thanks for the clue!
"I am not in a hurry.
I am retired.
And I don't care how big your truck is."