From MozillaZine Knowledge Base
This article deals with getting Firefox and Yahoo! Mail to play nice. See Yahoo if you want to use it with Thunderbird or SeaMonkey.
If you use the No Script extension, add about:blank to the whitelist.
If you have installed Yahoo! Messenger on your system, you may experience crashes when using Yahoo! Mail. This is caused by the Yahoo! Application State plugin (bug 322363, bug 326344). To see if this is the cause of your problems:
Type about:plugins in the Location Bar and press Enter. If you find "Yahoo! Application State" ("npYState.dll") in the installed plugins list, exit Firefox, open the C:\Program Files\Yahoo!\shared folder and rename the file "npYState.dll" to "XnpYState.dll" to disable the plugin. Open Firefox and test Yahoo! Mail.
If you see the following plug-in instead: "Yahoo! activeX Plug-in Bridge (npyaxmpb.dll)", delete or rename this file to solve the problem: C:\Program Files\Mozilla Firefox\plugins\npyaxmpb.dll
Incompatible Operating System
In the AT&T Yahoo! Mail Beta, the browser may display the following message: "Looks like your operating system is incompatible." To bypass this message, go to http://us.f819.mail.yahoo.com/dc/launch?sysreq=ignore