These are just some solid Firefox extensions that I use.
1. Download Sort: One click downloading and automatic organization into folders based on file types. This single handedly made one browser interwebs possible for me.
2. Adblock: It blocks advertisements, what more can I say?
3. FoxyTunes: A media player script extention for Firefox that allows you to control iTunes, Winamp, Windows Media Player, and much more from your browser.
4. SwitchProxy Tool: This really makes proxy browsing work, especially with the Tor client.
5. NoScript: Blocks unknown scripts from running on your browser. This is a must for those concious of security threats.
Finally, though I don't use FlashGot, I heard it was good; try it for yourself.