1. Close FireFox
2. Copy nppdf32.dll from the adobe reader dir (with default install, It should be found here: C:\Program Files\Adobe\Reader 9.0\Reader\Browser)
3. Paste it in your plug-ins dir, overwriting the one that is there (with default install, the plug-in dir is here: C:\Program Files\Mozilla Firefox\plugins
4. restart Firefox and try it out