If you're like me you have a ton of bookmarks. Some you go back to frequently and some you only use every so often. The problem is keeping them in sync on all your computers or remembering how to Google the page back. The answer is simple and easy to get used to. The answer is
Foxmarks. If you don't use this extension go grab it. Try actually organizing your bookmarks and use ctrl-b to bring them up, and quickly search through them.
While I'm talking about Firefox extensions, if you are a web developer you have to try out
Firebug.
For a more complete list of useful extensions, check out this
list.