Imagine you’re sent to a remote island and you’re only allowed to take one Firefox extension: which one would it be?
For web developers, it can only be the aptly named web developer extension: whether you’re using it to test your own pages or to find out how someone else designed his website, this extension is extremely useful. The only thing I miss is the ability to disable frame support, but this feature is already on the to-do list (if you need it badly, Opera has an option to disable frames).
Not a web developer? Use Foxclocks to display the time in different cities around the world or Crash Recovery to automatically restores all opened pages after a crash happened (useful if you behave like me and always open new tabs instead of bookmarking interesting websites right away).