I am trying to view a PDF document in my browser (Firefox 3.6 for Windows XP), like the Adobe Reader does, but using Sumatra PDF instead. However, when I try to do this, it comes up in a separate window. Is there any way to make the PDF come up in the browser instead?
Not with Sumatra PDF.
If you want a browser plugin, you could use the light and free (some advanced features require the not-free Pro version) Foxit Reader.