Many times quitting your browser completely, re-opening it and going back to http://www.pandora.com can help.
If you're still having issues using Pandora in Firefox, try the following steps:
1. Update Firefox: http://www.mozilla.org/en-US/firefox/update/
2. Clear your Cookies, Cache, and History.
3. Disable Adblock, Flashblock, and NoScript Extensions.
- Click on Firefox and select Preferences
- Select the Privacy tab.
- Under History, next to Firefox will, make sure it's set to Use custom settings for history.
- Make sure Accept cookies from sites is checked, and click on Exceptions.
- Enter *.pandora.com in the Address of the website field and click on Allow.
- Click Close on that menu and then click OK to close the Preferences menu.
5. Install the latest version of the Flash player: http://get.adobe.com/flashplayer/
6. Try opening Pandora in an alternative browser like Chrome to see if you have any more luck.
7. Finally, if the issue persists, try rebooting your computer and then returning to http://www.pandora.com