I ran into a situation with a client the other day that reminded me how dangerous it is to have just one browser application installed on your computer.
With this particular client the problem was that Internet Explorer suddenly stopped working. At all. Every time she launched it it shut down immediately. It apologized first and offered to report itself to Microsoft, but still.
At that point it became clear that we were in real trouble, because the easy solution would have been to update IE or to download another browser. But we couldn't. Because we needed a working browser to do either.
It eventually got all sorted out, and she now has both Firefox and Chrome installed, but the experience put the fear of god into me. Everyone should have an extra browser or two around for emergencies. Download one now.