I have no problems with Firefox 11 on Mac. Could be your preference settings.
Open a Finder window, select Go to Folder in the Go menu, and type in ~/Library
In the Preferences folder, find org.mozilla.firefox.plist and Move it to the Trash.
Start Firefox back up and let it rebuild the preferences. Try your DSM.
DS 1512+ w/3GB, 5 x 3TB Seagate ST3000DM001 8.2TB RAID 6, half files/half Time Machine.
Icy Dock MB559U3S-1SB enclosure w/4TB Hitachi UltraStar via USB 3 for files backup
UVERSE to AirPort Extreme + 2 AirPort Express w/speakers. TRENDnet TV-IP312WN camera
CyberPower CP1500PFCLCD Sine Wave UPS
DSM 4.1-2661 w/SSH + SFTP, VPN Server, Syslog Server, Media Server, Mail Server, Mail Station,
Audio Station, Surveillance Station, Photo Station, Web Station - DS Apps on iPad & iPod Touch.