Nicky,Firefox 5 ?
Fogive my ignorance,but Firefox has only just released the Firefox 2.0 Beta2 version.Did you mean version 1.5 ?
This may help you taken from click here
Do not tell us or others that "FlashGot doesn't work" if you have no download manager installed or, worse, if you don't know what a download manager is. If it is not clear enough, let's put it in bold letters: in order to use this extension, YOU REALLY NEED AN EXTERNAL DOWNLOAD MANAGER (pick yours in the dozen listed above).
FlashGot was born only to integrate Firefox with external download manager programs: it's quite logical that if you have no one installed, FlashGot won't work!
Hope this helps.