Steam's browser and chat windows are now grouped as tabs! You can manage multiple chats and web pages each in a single window in our latest Beta.
We've also been making some improvements to Steam's browser performance and stability, with the biggest gains on the Mac in particular. Flash video playback is now faster and more stable here, and the browser now loads cached content quickly. The video below shows a before and after comparison of our store's home page loading in the released Steam client and our latest Beta. The comparison is fair, in that both versions of the client had the same amount of time after start up to "settle," and both had the page content in their local cache. What was once molasses is now a normal web page in a normal-speed web browser. Yay!
You can give these improvements a whirl in the latest Beta by visiting Steam client Settings or Preferences and opting into the Beta Update offered there. And as always, do share your feedback on the Steam Forums.