(This is only suited for those who use Gmail but not Yahoo Mail)

If like me you are annoyed that the built-in Flock mail checker can only check one account at a time, and checking another involves logging out of the first, here's a tip - install the Firefox extension Gmail Notifier and set it up for multiple Gmail accounts. By default it puts its icon on the status bar, but if you want you can remove this and then use the "customize" menu to replace the Flock mail button with the extension's own button, like this...

Sorted!