Firefox is storing the Yahoo page in its cache, and reloading the 'old' version when you revisit the site. You can update the page at any time by clicking the 'Reload' button. (It's the blue circular arrow).
To ensure that Firefox gets rid of the cached version whenever you close the browser, do this:
Go to Tool>Options.
Click on the 'Privacy' tab.
Use the drop-down menu to select 'Use custom settings for history'.
Click to place a tick alongside 'Clear history when Firefox closes'. (Tick anything else you might want to select as well).
Click 'Settings' and place a tick alongside 'Cache'. (Also tick anything else you want to delete when Firefox closes).
Click 'OK' twice.
Chris