Re: ttps://accounts.google.com/CookieMismatch - I have followed the help guide but this still appears when I try to access my home page - any ideas?
It sounds as though you might have tried tried these, but just in case: When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.
(1) Bypass Firefox's Cache
Use Ctrl+Shift+r to reload the page fresh from the server.
(You also can clear Firefox's cache completely using:
orange Firefox button or Tools menu > Options > Advanced
On the Network mini-tab > Cached Web Content : "Clear Now")
(2) Remove your google.com cookies (save any pending work first) using either of these:
While viewing a page on the site, right-click and choose View Page Info > Security > "View Cookies"
Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies"
Then try reloading the page. Does that help?
Two other recent threads on this error on Gmail or Google login: