Probably one of my favorite features of Firefox is its very intuitive incremental search. In fact, I find it so natural that I often find myself trying to use it in other programs. Luckily, the other program I spend most of my time in (Flex Builder 2) also supports it.
Just type CTRL-j and start typing your search term. The focus in the editor will move to the match as you type. Type CTRL-j to move to the next match, and ESC to exit. The search status is also displayed on the status bar.
What is your favorite Flex Builder keyboard combo? Post them in the comments?