This is a known bug, and has been for years. It occurs when Firefox is your default browser and some glitch (or program) changes that default. Thankfully, there's a fairly easy fix, though it does involve monkeying around in the Registry.

(Note: These instructions apply to Windows Vista and 7 only.)

1. Click Start, type regedit, and then press Enter.
2. Navigate to the following entry: HKEY_CLASSES_ROOT\FirefoxURL\shell\open\ddeexec
3. In the center pane, double-click the Default entry, then remove whatever value is there (it'll probably be a weird string of numbers and commas).
4. Click OK, then exit Regedit.
5. If Outlook and/or Firefox are open, close them, then restart them.