Rated 5 out of 5 stars

Great little util.
Will work with FF3, but you have to:-
(1) Save 'date_picker_calendar-1.0.82-fx.xpi' to your disk.
(2) Uncompress the file (it's a zip file without the extension)
(3) Edit install.rdf with notepad (search for 2.0.0 and replace with 4.0.0) this number is the latest supported version this add-on works with (tested and installed ok with FF3 v3.0).
(4) Recompress files and folders into date_picker_calendar-1.0.82-fx.xpi (may need to comress as date_picker_calendar-1.0.82-fx.xpi.zip then rename to date_picker_calendar-1.0.82-fx.xpi)
(5) Point your browser to the location of date_picker_calendar-1.0.82-fx.xpi and click on the file (i.e. if you have the file in c:\addons, place this into your URL/Address bar.

Should now install on FF3 and on restart be in your address bar.

This review is for a previous version of the add-on (1.0.82).