I'm getting acroread instead of evince even though it says it's going to use evince

[Chris Green]

When I click on a .pdf file in a web page Firefox tells me it's loading
it and beside the selected 'Open with' button it says "/usr/bin/evince
(default)" but if I then click the OK button the file gets opened in
acroread. 

/usr/bin/evince is installed and works perfectly OK.

How do I get Firefox to use evince?

-- 
Chris Green