Many site issues can be caused by corrupt cookies or cache. In order to try to fix these problems, the first step is to clear both cookies and the cache. Note: This will temporarily log you out of all sites you're logged in to. To clear cache and cookies do the following:

1. Go to Firefox > History > Clear recent history or (if no Firefox button is shown) go to Tools > Clear recent history.
2. Under "Time range to clear", select "Everything".
3. Now, click the arrow next to Details to toggle the Details list active.
4. From the details list, check Cache and Cookies and uncheck everything else.
5. Now click the Clear now button.

Further information can be found in the Clear your cache, history and other personal information in Firefox article.

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I can neither reply or send new email. Twice I have disabled all plugins which corrected the problem temporarily but it keeps happening again. When I try to reply I get a message about original email found but unable to load. When I try to send new mail my keyboard simply does not type. I can open Internet Explorer and reply/send new email. I do not want to go back to IE.

hello, in this thread a user has found out that the problem might be related to the ghostery addon - do you have it installed too?

if so, add an exception for yahoo mail: click on the ghostery icon while you're in yahoo's webmail, choose "edit blocking options" & then don't block on this domain...