Highbrow hangs out in your menubar and has three separate behaviors. You select the one that will best suite your style. If you click on the Highbrow icon in your menubar, then you'll see the three options listed under "Behavior".
When using this option, your Mac will feel pretty much the same way it did before you installed Highbrow. The only difference is that you can now change your Mac's browser at will from the menubar by selecting a different preferred browser.
This option really gets Highbrow working for you. Now whenever you switch from one browser to another, your Mac will use the new browser. Thus, if you use Safari, but launch Firefox then your Mac will use Firefox to open links. Switch to Safari again and now your Mac knows to use Safari to open links.
When using this option, your Mac will present you with a list of browsers to choose from whenever you click on a link: