An interesting scenario was presented by a Proof-of-Concept exploit that uses IE(any version) to run possible malicious Javascript codes in Firefox version 2.0.

Yes, dear readers, IE and Firefox working together! Aptly categorized as Cross Browser Scripting, this particular exploit targets vulnerable parameters passed across different browsers (in this case IE to Firefox). As of the moment, several Proof-of-Concept codes can be found in web, where safe payloads such as spawning a command prompt or saving a non-malicious text file are performed.
This does not bode well and I think it is only a matter of time when malware authors start developing actual malware codes based on this vulnerability.

To add more “bad” news, Microsoft (IE) and Mozilla (Firefox) are pointing fingers on who should fix the flaw.