Type-ahead find is a quick way to navigate around in a document using only the keyboard. Even though the name suggests using it for text searching, many people also find it useful for moving the cursor to a place they already see, much faster than by using the arrow keys.
With the cursor somewhere in the browser window [Ed. note: this needs to be clearer!], simply type a few characters of the link that you wish to go to, and the cursor will snap to the first link that contains those characters. If you mistyped a character, simply type backspace and type the correct one. To cancel a find process, you can either hit Esc or do something else.
Normally, this only searches links; type a "/" character first to search through all text. To find the next instance of your search, either type Ctrl-G, F3, or the last letter of your search (unless there is a match that includes the repeated letter, in which case it will go there first). Shift-Ctrl-G or Shift-F3 will find the previous instance.
As an example of this, suppose that you're in a web page that contains links with the words "could", "coanda effect", "cool", and "coxwain" in them. Typing "co" with the cursor at the beginning of the page will select the "could" link, typing an additional "o" (making "coo") will bring the cursor to "cool", and typing "o" additional times will cycle through all four of the links.
Of course, this doesn't work if you're in an edit box in a form; type the Tab key to get out of the edit box first.
[Note: before 1.0 release, check http://www.mozilla.org/projects/ui/accessibility/typeaheadfind.html and see if any of the to-do items -- specifically, Ctrl-' and Ctrl-/ -- have been done!]