Are you clearing the cookies via Delete browsing, search and download history on Firefox?

If you do then you can try to exclude the cookies to see if that helps.

If you use Delete browsing, search and download history on Firefox to clear the cookies when you close Firefox then Firefox also tries to clear the cookies from plugins like Flash and starts a plugin-container process with the Flash player and possibly other plugins as well for each cookie, so that can add extra time if there are a lot of cookies to remove.
Also make sure that all plugins are up to date.

You can let all cookies expire when you close Firefox.

• Tools > Options > Privacy > Cookies: "Keep until": "I close Firefox"

Hi Cor-El (You related to Superman's dad?) Sometime clear recent history but that had no -ve effect when using FF4. This problem specifically started on loading FF5 beta. There was a warning message that some add-on are not compatible with that version. As far as I can tell, once the error message appears it stays there until it is cleared. i.e. it is not a question of being impatient whilst cookies are being gathered. Also, more annoying that the first occurrence of the error message, ticking the "do not show this message again" box has no effect and every time a new screen opens the error message appears, but only after hitting escape, flipping tabs etc. Deleting my profile and starting from scratch also made no difference. Going back to FF4 did. ???