Easy, you open up the one YOU want to use. Simple as
Or if you want to have Both opened up, open them both up and just click on the window of your browser to make it active. It's one of the easiest things to do.
I have on my Mac...
Safari (which is what I always use)
Firefox (I forget I have it LOL)
Explorer (which I never use cause that shit freezes more on me than a Russian winter LOL)