If you still want to use the old Facebook you can -- for the time being, anyway.
Dhanik Modi, a student at Virginia Tech, discovered this weekend an issue that causes Facebook to revert to the old version under Firefox 3.1 alpha (code named Shiretoko).
Example 1 (click on image to expand)
Example 2 (click on image to expand)
The upper left frame of in the browser shows a message that says, "Sorry, the new Facebook is temporarily disabled." Both Mozilla and Facebook are apparently working on a quick fix for this but, in the meantime, if you'd like to extend your old Facebook experience a little longer you can do so using Firefox 3.1 alpha.
Update 09/15/08 10:30 a.m. - Also, someone has created a Facebook app that switches profiles over to the old version of the social network.