In Firefox (at least version 18.104.22.168 on the PC, I haven’t confirmed this on others) you can search for text by simply typing a slash (/). As long as you’re not in a text entry box the / character will open up the find dialog box just like control-f would.
For those of you who don’t understand the significance of this, in the popular UNIX command line text editor vi you search for text by typing
/string to find. This is typical of how vi works: powerful, simple commands which are rarely obvious.