There was added a classification of an HTTP request transaction in bug 599164 to group similar requests on a single connection and pipeline. The code that decides on the class [1] is not very clean, class is based on a vague check for Accept header value or very simple URL parsing. Consumers must be able to explicitly set the class on a channel using the new API. This API can also be used for much simpler pre-test implementation (bug 603505). My first suggestion is to add an attribute to nsIHttpChannelInternal called PipelineClass. Channel then propagates the class to the transaction. Image loader, script loader, css loader will then explicitly set the class. Default value will be GENERAL. Channel will have the right to reset to REVALIDATION, but only if class is not PRETEST. [1] http://hg.mozilla.org/projects/larch/file/f5e03f4cb36f/netwerk/protocol/http/nsHttpTransaction.cpp#l156