Try this knowledge base article: Third-party cookies and Firefox tracking protection

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes). See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears. You can use "Disable all add-ons" on the Safe mode start window. You have to close and restart Firefox after each change via "File > Exit" (on Mac: "Firefox > Quit")

There's a very cool extension called CS Lite that lets you manage which cookies to allow and disallow very easily and with whitelists and blacklists and stuff. Very cool.

after trying the cookie managers and this one was last firefox will not close, my home page will not load, gives a restore last session error, and none of the cookies are deleted when I close firefox now even though I uninstalled this and the others and reset the settings back to until I close firefox.