This information is obsolete; I haven't worked on GIMP for a long time.
Web Browser GIMP extension is not an HTML browser (at least, not yet ;-); it's just an interface to an external web browser that allows GIMP, using a PDB call, to instruct a browser to open a URL.
Currently, it works with Netscape, version 1.1 or above, using a protocol described in detail on this page. If Netscape is not running already, the extension starts it, with a URL as a command-line argument.
Screenshots: