+1 for Download Helper and NoScript.
I also use ...
FoxClocks, as I do business with several different countries and it gives me instant mouseover view of the local times in each without using up any screenspace.
FireFTP, which saves having separate FTP software.
Greasemonkey, with a bunch of scripts.
Skype Extension, to give single-click dial of phone numbers on webpages.
Finally, three really simple yet useful add-ons for the terminally lazy like me: Go To Google, New Tab Homepage and TinyURL Creator.