From MozillaZine Knowledge Base
HTTP is the application-layer protocol with which most web pages are transferred. As part of the connection between client and server in HTTP 1.1, the client lists in what way it can/prefers to accept information (content negotiation). This preference controls which MIME types the application will report it supports.
Possible values and their effects
A comma-seperated list of MIME types with optional "q" (quality) values. A quality value says that the application prefers one type over another, but supports both. Quality values can be from 0 (not supported) to 1 (preferred format). If a quality value is omitted, it defaults to 1.
All others default:
Default in trunk builds after 2007-03-08:
First checked in
Has an effect in