Video source: http://www.youtube.com/watch?v=VIVIegSt81k < math video Video works fine in HTML 5 playback but chops between significant frames when using flash Application Basics Name Firefox Version 18.0a1 User Agent Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:18.0) Gecko/18.0 Firefox/18.0 Build Configuration about:buildconfig Extensions Name Version Enabled ID Important Modified Preferences Name Value browser.cache.disk.capacity 1048576 browser.cache.disk.smart_size_cached_value 1048576 browser.cache.disk.smart_size.first_run false browser.places.smartBookmarksVersion 4 browser.startup.homepage_override.buildID 20121001030603 browser.startup.homepage_override.mstone 18.0a1 dom.mozApps.runUpdate false extensions.lastAppVersion 18.0a1 network.cookie.prefsMigrated true places.history.expiration.transient_current_max_pages 104858 privacy.sanitize.migrateFx3Prefs true Graphics Device ID 0x6741 GPU Accelerated Windows 1/1 OpenGL Vendor ID 0x1002 WebGL Renderer no information AzureCanvasBackend quartz AzureContentBackend none AzureFallbackCanvasBackend none JavaScript Incremental GC true Accessibility Activated false Prevent Accessibility 0 Library Versions Expected minimum version Version in use NSPR 4.9.2 4.9.2 NSS 3.13.6.0 Basic ECC 3.13.6.0 Basic ECC NSSSMIME 3.13.6.0 Basic ECC 3.13.6.0 Basic ECC NSSSSL 3.13.6.0 Basic ECC 3.13.6.0 Basic ECC NSSUTIL 3.13.6.0 3.13.6.0

I think this is bug 795796, so it should be fixed in tomorrow's nightly.

The problem is still happening on today's nightly.