Recently in the last few months many people on the Web have complained that they can’t sign in to My Yahoo using Firefox without having to clear their cookies every time. I, too, suffered from this problem and simply used SeaMonkey (another Mozilla browser) because I got annoyed and didn’t spend time to figure out the solution. Well, I finally devoted some time to figure out what happened to my beloved Firefox. Apparently my cookie files became corrupt. Once I deleted the cookie files (not just the cookies from the browser), Firefox automatically re-created the cookie files and now all is well. If you are having the same problem you might want to try this solution. You may want to copy-and-paste your cookie files as a backup before you delete.
Your Firefox cookie files are located here:
C:Documents and Settings[USER]Application DataMozillaFirefoxProfiles[GOBBLEDYGOOK].[USER]
Delete both cookies.sqlite and cookies.txt (if you have both)