Are you sure that the Youtube videos are still auto buffering when paused? That used to be the default but Google changed it awhile back when they switched to adaptive streaming. I just tried it (on both Chrome and Firefox) and the buffering only occurs while playing. If you change the quality of the video, buffering may occur on pause but that is apparently not the default.
You may want to try something like the Chrome Flashblock extension
. This will block Flash from automatically loading on any site unless you click on the flash icon in the Flash content area or add the site to a white list of sites where Flash is allowed. While it doesn't do exactly what you want, it could help you use less bandwidth. There is a Firefox Flashblock add-on
but I haven't tried it so can't vouch for it.