hello nearel, please try the "Clear history for that site" step from the following article Websites say cookies are blocked - Unblock them (but watch out - this will also delete any passwords saved for those sites in firefox).

other causes of the problem may be security software or extensions/third-party programs that interfere with the cookie management in firefox... http://kb.mozillazine.org/Websites_report_cookies_are_disabled

Did you check the cookie exceptions to make sure that you aren't blocking cookies from those sites?

• Tools > Options > Privacy > Cookies: Exceptions

You can also check the cookie permissions on the about:permissions page via the location bar for websites.

You can Create an allow cookie exception to keep cookie from a website if you want the website to remember you and keep you logged on.

• http://kb.mozillazine.org/Cookies

or you can write in firefox passwords for your sites that you want to login and firefox will automaticly login.... or login trough other browser and change security settings for log in from difrent computers... you see, for example, Google chack your IP adress and itch browser you are using when you try to login... probably if you have high security setting he think that you are intruder ;) got to google accoutn and turn off checkin and remembering computers from which you are login in. same problem some people have with facebook... and some people who change passwords and did not made changes to their connected apps and browsers... try it, it doesnt cost you anything... write mi if you need more data...