From MozillaZine Knowledge Base

This article deals with getting Firefox and Yahoo! Mail to play nice.

Blank messages

If you use the No Script extension, add about:blank to the whitelist.[1]

Crashes

Yahoo! Messenger

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

Firebug

The Firebug extension can also cause crashing when users log out of Yahoo! Mail.[2][3]

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