In previous version of UFT you could select the target browser to be used for the run sessions from: UFT > Run > Record & Run Settings > Web tab from the available drop down.
Now with UFT 12.50 we included an easier and more flexible extra option to do this:
Then you can define on the DT the browser that will be used to run the test. As you can think, yes, you can use a different Browser for each different iteration of the test if you want.
For additional information about the full set of variables names and descriptions, please refer to:
UFT Add-ins Guide > Working with UFT Add-ins > Web-Based Application Support > Environment Variables for a Web-Based Environment