Playing an h264 encoded (in mp4 container) file directly in the browser on FF 30.0 with hardware acceleration turned on produces the following behavior for me on Windows 7: 1st play - fine reload page - video will not play play any *other* h264/mp4 video (or the same video in a second tab) - video will not play close tab, clear cache, open new tab with video - video plays again, but no further video or reload will work until procedure is repeated. turn off hardware acceleration, restart Firefox - everything behaves as it should. (Though cpu usage is now through the roof, of course.) I've tried this with all addons disabled, in safe mode, and with a totally blank profile - same behavior. Here's a test video: http://www.quirksmode.org/html5/videos/big_buck_bunny.mp4