Is it possible to use a NPAPI plugin to filter HTTP traffic in modern browsers (namely firefox and chrome)? I was able to build a simple plugin and register it with FF and Chrome.
Is there any way to use it for low level HTTP filtering? I couldn't find any useful information on the net about this.
No. NPAPI plugins don't affect the browser or browser stack; they affect the handling of a specific mimetype or they affect a specific page into which they are inserted as an object tag.