Before trying the uninstall and reinstall Firefox I have a question or 2 to ask you.
What version of Yahoo mail are you using?
Yahoo! Classic? Did you ever upgrade to the newer version of Yahoo! offered?
I also found one of your older threads where you were still using Firefox version 3.6.18. Are you still? I have FF 6.0.1. Thought about reverting back to 3.6.20 myself which is the last version I had prior to upgrading to FF 6. But it doesn't seem to be feasible if that is what you have.
I see that you had posted this same issue on June 06, 2011. -->
Using the File/Send Link commandReason I ask.......
Whilst giving you the instructions in my first post on how to set your default email in Firefox to Yahoo! I went ahead and did the same on my computer (which I have been wanting to do for some time but never took the time to do).
At first everything worked great! Then just today when I clicked File > Send Link I got a URL that was unclickable so I ventured out looking for an answer to no avail. I reverted back from the newer version of Yahoo! to the Classic version of Yahoo! and lo and behold the URL went to the
To: field and not the message body of the email. So actually I simulated your issue by experimenting trying to correct my own.
I have found a plethora of people who have this problem yet no solution. So upgrading to the newer version of Yahoo! will be of no use either. This is more than likely a Yahoo! problem versus a Firefox problem. I did find that many are not having this same issue with IE.
Following is a link that I found though I haven't taken the time to research/experiment with the content.
Using Firefox with Yahoo mail: How do I make "clickable" a url that is pasted into the body of an email?So what this boils down to is, Yahoo! may no longer be compatible with Firefox as a default email.

I hope my efforts have helped in some way.
Edited by DonnaB, 04 September 2011 - 09:39 PM.