If you've upgraded to a newer version of Firefox, you have to update your add-on too. A compatible add-on which works with new version. Download and install the latest version of your program (Flashget). This shall do the trick. (This requires that the company of your software produced a new version which is compatible with the new versions of Firefox) Also you can download and install 'Internet Download Manager' the latest version. It is compatible with all of the versions of Firefox till now and it's extension lets you to download streaming video/audio. If this helped don't forget to hit "Solved it" so the topic would be closed.