Yes, very simple to download and use. Just make sure you're logged into the Flight Memory website, and that you're on the 'Flight Data' section in your profile before clicking the Firefox plugin. It then downloads all your flights across all pages as a Microsoft Excel *.csv file.

For example, I just backed up all my flight data, by using this tool, from 'Flight Memory' to 'Log My Flight' but I don't see the former dying out any time soon!