On my system – Windows XP SP2, when I clicked on a link in Outlook 2007 the link would open in IE which is not my default browser – Firefox is. The links would open in IE even if Firefox was open.
I was able to tweak my system as listed here to have the links open in Firefox:
1. Fire up My Computer
2. On the menu go to Tools -> Folder Options
3. Go to the File Types tab.
4. Find Extension: “(NONE)” File Type: “Internet Shortcut” and click on it once to highlight it.
5. Go to the Advanced button.
6. In the Actions: box click open to highlight it.
7. Click the Edit button.
8. In the Application used to perform action: box browse to your firefox.exe location and click on firefox.exe and then click on open.
9. In the Application used to perform this action: it should read: ” your path \Mozilla Firefox\firefox.exe” -url “%1”
10. Click OK.
11. Find Extension: “(NONE)” File Type: “URL: HyperText Transfer Protocol with Privacy” and repeat the steps.
12. Also see if you have this Extension: “URL” File Type: “Internet Shortcut” and repeat the steps here too.
Links clicked on in Outlook now open in Firefox for me.