Updated version 18.104.22.168 now works with FF 3.6.13 Rated 4 out of 5 stars
The plugin version 22.214.171.124 was apparently updated in early February (2011), without changing the version number, in order to work with Firefox 4. I don't have FF4 but am happy to report that It works for me under Firefox 3.6.13 (using Windows Vista). Maybe it helps that my default PDF software is Nitro PDF Professional (albeit the somewhat older version 126.96.36.199.). It is a bit slow sometimes, but it works pretty well.