The Security Mentor

"...involving both Internet Explorer and Firefox", says one writeup.

What's happening is that if you have both installed, then visiting the wrong place with Internet Explorer can trick Firefox into doing the wrong thing.

While geeks argue about whose fault this is, you still need to protect yourself. Stay out of bad neighborhoods for one thing. Antivirus firm Symantec says "Disable the "Firefox URL" URI handler" but doesn't explain how. Slashdot user mario_grgic gives step-by-step instructions:

Open Windows Exporer (not Internet Explorer) and from the Tools menu select "Folder Options" menu. On the dialog that appears select the "File Types" tab.

Now in the list of registered file types find the one that says:

"(NONE)" for extension and "Firefox URL" for file type

Select it and click on delete button to delete it.
Click on "OK" to close the "Folder Options" dialog.

If you do that, the only change will be that some web sites will no longer be able to force themselves to open in Firefox. This is not a serious loss.

UPDATE 7/11:

You're already protected against this if you have installed the Firefox extension NoScript, according to NoScript's author.