404 error is not firefox problem. if you don't believe, try google chrome 10.x

Then what kind of problem is it? It could be related to Java or my anti-virus program.

404 and 500 errors are so common during internet rush hours, from dusk to mid-night, everyday, especially monday to friday. so, don't be that fussy. and make some more alternative for yourself, use I.E. 7/8/9, firefox 4/3.6.x, and google chrome 10.x, if you find the your browser now not proper, change it, you have the right to take your own good one.

This is a chronic problem. Not related to time of day. It is a permanent block on my ability to log into my Yahoo mail from the Firefox web browser on one computer ONLY. I can log in from other computers. This seems to have nothing to do with add-ons or extensions. Upgrading from Firefox 3.6 to 4.0 had no effect and this also does not effect any other web browser on the same computer.

Again, every time on try to log into my Yahoo mail using Firefox on my main desk time I get a "404 Not Found" error.

Please give me a serious answer.

Meant to say on my "main desk top".