OK zillah,

I mostly use FireFox, so I have the Yahoo! toolbar installed, and just click on the mail icon.

In Opera I just type Yahoo into the address bar and hit "enter" it takes me to a Google search page with the Yahoo! UK site on top (it seems to know what country I am in). I just click on the mail link, and it takes me to the Yahoo site.

I have just opened my mail account in both Opera and FireFox. 2x Windows 7 64bit and XP Pro 32bit, so it doesn't look Windows related?

Also, html files open just fine. FF is my default browser so the system uses that, unless I tell it to use something else.

It sounds to me as if you might have a corrupted FireFox installation. You might try reinstalling FF and if that doesn't work then uninstalling and reinstalling it.

REMEMBER! if you have to uninstall please save your Bookmarks (favourites) first

Good luck!