User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.3) Gecko/20070309 Firefox/2.0.0.3 Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.3) Gecko/20070309 Firefox/2.0.0.3 I was looking something up in the XHTML 1.0 DTD on the W3C website (URL provided). It asked me what application I wanted to open it in, and recalling a handy option in Opera that allows you to choose to view as text/HTML (bug 57342), I chose to try opening it in firefox.exe. Unfortunately, when Firefox is asked to open a file, it checks what application to forward it to rather than trying to display it as plain text or some other failure mode, so it started flashing in and out of focus and displaying other odd behavior. Reproducible: Didn't try Steps to Reproduce: 1. Go to URL (assuming you have no handler specified for .dtd) 2. Choose "Other..." and choose firefox.exe. Actual Results: Firefox shoots up to 100% CPU, grabs focus from other applications continuously, the tab flashes in and out of focus, etc. Closing the tab doesn't help, it reopens itself. Need to shut down Firefox, reopen it, and close the appropriate tab before it starts loading, then remove the association with Firefox. Expected Results: Either display as text (maybe HTML, if you're not worried about arbitrary scripts/applets being executed) or notice something's wrong and fail gracefully. Ideally the solution should work even if you try to open with another instance of Firefox, I suppose, if for some reason you have multiple binaries lying around. (I did search, but of course this may be a dupe anyway. If so, sorry in advance.)