Instead of adding code to the test to change browsers, you can add local test settings to the project's solution. Then use MSTest from the command line and call the desired setting with a switch.
In Visual Studio, go to the Solution Explorer in the upper right. Double-click "local.testsettings" and highlight "WebAii Test" on the left side. Change the Default Browser to Firefox. Click Save As and give it an easily recognizable name. You can repeat those steps for each desired browser, creating a new settings file for each.
When running MSTest from the command line, add "/testsettings:[file name]" to call the desired browser.
A video clip is located here
that clearly lays out these steps and provides screen captures to follow along.
the Telerik team