As the title says. If you use replaceWith() in the jQuery javascript library to replace something with an HTML5 or tag, in Firefox 3.6.0-3.6.3 this results in two copies of the video or audio on top of each other, playing simultaneously. If you pause the video or audio on top, the hidden one underneath continues playing. This may be a bug in jQuery rather than Firefox, but Firefox 3.5.x behaves as expected with the same code, and Webkit-based browsers also don't show this behavior. Tested with both and tags under Firefox 3.6.0 and 3.6.3 on Mac and 3.6.3 Windows, all same. It has also been reported as a jQuery issue on a descriptive page, but here are a couple of bare testcases in addition to the real-world page on DaringFireball linked as affected: http://www.bridgedev1.com/videotest.html http://steelbluepanic.com/html5testcase/testcase.html And the jQuery report: http://forum.jquery.com/topic/using-replacewith-to-insert-html5-audio-or-video-in-firefox-results-in-two-copies-of-the-inserted-object-on-top-of-each-other == URL of affected sites == http://daringfireball.net/misc/2009/12/user_guide_demos == User Agent == Mozilla/5.0 (Macintosh; U; PPC Mac OS X 10_5_8; en-us) AppleWebKit/531.22.7 (KHTML, like Gecko) Version/4.0.5 Safari/531.22.7