See this: http://support.mozilla.com/en-US/kb/Changing+the+e-mail+program+used+by+Firefox

Thanks for trying, but as I said, I already have Thunderbird set as the Mailto client in Firefox 4. What I didn't mention is I already removed Gmail as a selection in that option.

Also, following the link provided, Thunderbird is my email client and making the changes suggested and rebooting had no effect. Also, as I mentioned, this does not occur if I click the email link out of IE or Google Chrome, so it is likely something specific to Firefox and not to Windows XP.

Still looking for a fix!

Sorry, I didn't see mailto mentioned in your original posting.

Open up the Google Toolbar Options and look at the Search tab and see if you have Use GMail for Mail To links selected, under Web-browsing tools on that tab.

I have the same problem, and I also wondered if Google toolbar has anything to do with it. There GMAIL was set as the default mail application at first. I have removed that setting, but Firefox 4.0.1 continues to use GMAIL instead of Outlook which is my selection for mailto handling.