FYI. Here's a problem I was having and the fix I found, just in case any of you are having the same problem:

Installed Firefox and now when I click on a URL in Outlook, a dialog box appeared saying Locate Link Browser, even though Firefox had already launched. So I pointed the dialog box to firefox.exe and now 2 Firefox windows launch each time I click on a URL.

Here's the fix that worked for me:
(I didn't write the following part, so pardon the structure)

Double-click My Computer.
On the Tools menu, click Folder Options.
Click the File Types tab.
Under Extension, locate the "N/A" entry, and then locate the "URL:HyperText Transfer Protocol" entry under File Types.
Click Advanced.
Under Actions, edit Open and remove the checkbox tick in DDE, press OK. select Open
Click Set Default, and then click OK two times

do that for URL: HyperText with security
also and maybe for HTML HTM if still doesn't work. and you'll be fine