I just checked here with my Excel 2007, Firefox, and Windows XP Pro SP3, and Excel opened a link in Firefox, which is my default browser.
I would try "resetting" your default browser in the system by setting the default to IE and back to Firefox.
In Internet Explorer:
- Go to "Tools" and then "Internet Options"
- Click on the "Programs" tab
- Click the "Make default" button
- Click OK
Press Win+R to open the Run box, and type in a website (ie, www.google.com). It should open in Internet Explorer.
In Firefox:
- Go to "Tools" and then "Options"
- Click on the "Advanced" tab (on the far right)
- Click on the "General" tab
- Click the "Check Now" button
- If Firefox is not the default browser, which it shouldn't be right now, it should ask if you want to set Firefox as your default browser. Click Yes
Go back to the Run box (or press Win+R again) and type in a website (ie, www.google.com). This time, it should open in Firefox.
Check Excel and see if it's opening it in the right browser now.