I was recently asked what Firefox plugins I use. Here is my list:
AdBlock Plus - Block embedded HTML which pulls ads from other sites and slows your PC down.
ColorZilla - An eye dropper tool that allows you to get RGB or HEX values for any color on a web site.
CSSViewer - See the applied CSS styles on any HTML element by hovering your mouse over the element.
del.icio.us Bookmarks - Can't live without this! Store, share, and search your bookmarks across any computer.
DOM Inspector - View the Document Object Model of a web page visually.
DownThemAll! - Pull down any or all files on a web site (think multiple PDF's from a software vendor's web site).
FireFTP - GUI for FTP
IBM Software Support Toolbar - Search the IBM forums and support feeds.
Map+ :Map an address to Google maps based on the address format
MeasureIt - Measure the pixel width of object or space in a web page
Sage - An RSS feed reader
Skype Extention for Firefox - Turn phone numbers into clickable, callable links from Skype.
Web Developer - The greatest tool second only to Firebug for testing, debugging, and reverse engineering a web site.