I've not had the need to do this yet but one way is to use the Environment field. It's a custom field you can populate. It's visible in the test cycle player and could be used to execute a test against different environments (browsers in your case) by selecting Chrome for the first test, then the tester would click the "Start a new test execution" link, change the environment to Firefox and execute the test step. I think that would work but you'd need to check how that would look with your current reporting setup - you might find that you'd want to create two of each test.