I run integration tests in Ruby on Rails with Capybara using both the Chrome and Firefox drivers. That project was stopped for a while and when I went back to it the Firefox driver didn't work anymore. I didn't have the time to check what went wrong and fix it but I'm developing with Firefox anyway. I think it's important to test with the major browsers. My customer didn't ask about IE. I remember when I had a few VMs for that and its incompatible version tied to a specific version of Windows.