User Agent: Mozilla/5.0 (Windows NT 6.1; rv:42.0) Gecko/20100101 Firefox/42.0 Build ID: 20150824004008 Firefox for Android Steps to reproduce: in this example Youtube 1.- Open Firefox Developer Edition Version 42.0a2 (2015-08-24) 2.- Open Youtube on browser 3.- Put any video to play Actual results: On UI it says An Error Ocurred but in console says Media resource mediasource:https://www.youtube.com/6a9abbee-fd4e-40c6-9231-1d683f4619c7 could not be decoded. on every video Expected results: the video it must be loaded

Are you running FF on Android? Because with FF DE on desktop (Win 7), I have no issue to play any videos on YT.

We can't help unless you specify the exact phone make/model.

If it is a desktop issue then it is probably because you're using Windows N or KN.

Running into the exactly the same problem under Windows 10 Pro N 64bits (What's wrong about Windows N versions?) Details frm about:support just in case it helps Firefox 42.0a2 20150903004012 aurora Mozilla/5.0 (Windows NT 10.0; WOW64; rv:42.0) Gecko/20100101 Firefox/42.0 Safe mode: false Extensions ----------- None Gráphics -------- hardware H264: false GPU 1: igdumd64 igd10umd64 igdumdx32 igd10umd32 GPU #2): nvd3dumx,nvwgf2umx,nvwgf2umx,nvwgf2umx nvd3dum,nvwgf2um,nvwgf2um,nvwgf2um Intel(R) HD Graphics NVIDIA GeForce GT 420M Direct2D: true DirectWrite: true (10.0.10240.16430) Encuadre/zoom asíncrono: ninguno WebGL: Google Inc. -- ANGLE (Intel(R) HD Graphics Direct3D11 vs_4_0 ps_4_0) GPU: 1/1 Direct3D 11 (OMTC) driver version : 8.15.10.2900 dirver version(GPU #2): 10.18.13.5582 windowLayerManagerRemote: true AzureCanvasBackend: direct2d 1.1 AzureContentBackend: direct2d 1.1 AzureFallbackCanvasBackend: cairo AzureSkiaAccelerated: 0 Important modified Preferences ------------------------------------ accessibility.typeaheadfind.flashBar: 0 browser.cache.disk.capacity: 358400 browser.cache.disk.filesystem_reported: 1 browser.cache.disk.smart_size.first_run: false browser.cache.disk.smart_size.use_old_max: false browser.cache.frecency_experiment: 4 browser.download.importedFromSqlite: true browser.places.smartBookmarksVersion: 7 browser.sessionstore.upgradeBackup.latestBuildID: 20150903004012 browser.startup.homepage_override.buildID: 20150903004012 browser.startup.homepage_override.mstone: 42.0a2 browser.tabs.warnOnClose: false dom.apps.reset-permissions: true dom.mozApps.used: true extensions.lastAppVersion: 42.0a2 gfx.direct3d.last_used_feature_level_idx: 1 gfx.driver-init.appVersion: 42.0a2 gfx.driver-init.deviceID: 0x0046 gfx.driver-init.driverVersion: 8.15.10.2900 gfx.driver-init.feature-d2d: true gfx.driver-init.feature-d3d11: true gfx.driver-init.status: 2 media.gmp-eme-adobe.lastUpdate: 1441316832 media.gmp-eme-adobe.version: 12 media.gmp-gmpopenh264.lastUpdate: 1441316833 media.gmp-gmpopenh264.version: 1.4 media.gmp-manager.buildID: 20150903004012 media.gmp-manager.lastCheck: 1441316828 media.hardware-video-decoding.failed: true network.cookie.prefsMigrated: true network.predictor.cleaned-up: true places.history.expiration.transient_current_max_pages: 100357 plugin.disable_full_page_plugin_for_types: application/pdf plugin.importedState: true privacy.sanitize.migrateFx3Prefs: true

Are they these versions without WMP? Maybe the H264 Video Decoder is missing from Microsoft Media Foundation used by Firefox to decode H264.

You are right, installing https://www.microsoft.com/en-US/download/details.aspx?id=48231 solved the problem.

Jesús, if you don't have the Media Pack installed on your machine, could you install it and test again, please. Win 10: https://www.microsoft.com/en-US/download/details.aspx?id=48231 Win 8.1: https://www.microsoft.com/en-US/download/details.aspx?id=40744 Win 8: https://www.microsoft.com/en-US/download/details.aspx?id=30685 Win 7: https://www.microsoft.com/en-US/download/details.aspx?id=16546

Bug 1210231 should fix this.