I had some difficulty accessing my porn....err.....Yahoo mail. 
I wrote to Yahoo and within 24 hours (OK, perhaps it was a wee bit longer) they provided me with sufficient information to determine that the Firefox add-ons were to blame. 
I paste their response as it might be of use to someone else as well. 
....
Hello cjm, [
note on what personal terms we are! 
]
Thank you for contacting Yahoo! Mail Customer Care.
I understand you are having difficulty with your Yahoo! Mail account. I
apologize for any inconvenience this is causing you.
Based on the information within your email, I'd like to offer a few
troubleshooting steps outlined below that you can try from your
computer. After you complete each of the steps you will want to test to
see if the issue is still present. These steps are available with
detailed instructions in the Yahoo! Mail help pages located at:
http://help.yahoo.com/l/us/yahoo/mail/yahoomail/technical/browser-01.html- Disable any Internet Browser Add-ons or Plug-ins.
- Temporarily disable any security software.
- Delete your Internet Browser Cookies and Cache.
- Update Flash Player.
- Switch versions of Yahoo! Mail. Switch between All-New Yahoo! Mail and
Yahoo! Mail Classic versions.
- Ensure that JavaScript is updated and enabled in your Internet
Browser.
- Restart your internet browser.
- Restart your computer.
- Temporarily switch to a different supported Internet Browser.
- Test from another computer, please include the results of this test
with any response to this email.
- Test from another location, please include the results of this test
with any response to this email.
If the troubleshooting steps listed above do not resolve your issue, you
can either reply to this email including the results of any tests with
the response or you can also chat with us LIVE using Yahoo! Mail Live
Chat Support! Just copy the following link and paste it into the address
bar of a browser:
http://help.yahoo.com/l/us/yahoo/mail/contact_chat/forms_index.htmlThen just click on the "Chat with us live now" button, and you'll be
connected to one of our Yahoo! Chat agents.
I hope this answers your question. If you need any further assistance,
please feel free to contact us again and we'll be glad to help.
Thank you again for contacting Yahoo! Mail Customer Care.
Regards,
Bob
Yahoo! Mail Customer Care
Visit our online help pages at:
http://help.yahoo.com/help/us/mail/