This window allows you to surf the web from within the viewer. When clicking on a web link (URL) in SL, the page will open either in this window, or in your default external web browser application. You acn select which of these you prefer, from Preferences → Network & Files -> Connections → Web browser.
You can also open this window manually: Develop → UI → Web Content Browser (Ctrl-Alt-Q to enable to Develop menu, if you do not have it enabled already).
Use this window much as you would any web browser.
See this page for documentation on Firestorm 4.6.9 (42969) and earlier.