Vista favoring IE over other browsers?

I have an interesting problem. I recently bought a brand spanking new Dell with Vista. (No, that's not the problem). I set the default browser to be Firefox, being an IE hater. Everything worked fine for a while. Then, 2 weeks ago, Microsoft put out an update for Vista. Ever since that update, the computer refuses to let any program (including Vista itself) open a hyperlink in a new browser window. (For example, it will not open a hyperlink in an email). There are two exceptions to this problem.

1. Firefox will still open hyperlinks just fine.
2. The program specifically calls on IE (instead of the default browser) to open the hyperlink.

This problem even includes hyperlinks imbedded in the programs themselves, (i.e. links to the company website in the help menu, or Vista's "get help online" link). It extends so far that when right-clicking a hyperlink, the option to "open in a new browser" isn't AVAILABLE. (It's not grayed out, is simply isn't even there). While I can copy the link and paste it into firefox's address bar, that only works if the actual hyperlink is displayed (which is not always the case).

I was wondering if anyone else had encountered this problem, and if there was a way to restore the functionality of my programs.