I recently installed FF 13 to test this new browser with Win 8 RP. I encountered 2 problems:
1) The Foxit add on encountered a problem and closed. Basically I could not get Foxit to run on FF 13.
2) Adobe Flash would not install. States could not be registered. I followed the T/S tips for Flash and it still would not install.
Since I am testing this browser for possible adoption, these 2 problems are causing considerable grief. At this point I would say that I will stick with IE and Chrome unless these 2 things can be solved.