Is the problem specific to MP4 media, or does it affect all HTML5 video, even WebM or OGG media?

To test WebM, you can scroll down in this page to the section titled "WebM Videos within the HTML5 <Video> Tag" where you should see a player: http://www.webmfiles.org/demo-files/

If it is MPEG specific, do you use an N or KN build of Windows? If so, you probably need the Media Feature Pack to get the components Firefox uses to decode MPEG media: https://www.microsoft.com/en-us/download/details.aspx?id=48231

It seems specific to Youtube. I've run tests for MP4, WebM (including the link you posted) and OGG elsewhere and all work fine. Youtube just seems insistent on using Flash on my W10/more recent machine.

My Windows is straight up 10 Home, not K or KN.

What do you see on the YouTube HTML5 page?

• https://www.youtube.com/html5

Can you attach a screenshot of this page?

• http://en.wikipedia.org/wiki/Screenshot
• https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

• Use a compressed image type like PNG or JPG to save the screenshot
• Make sure that you do not exceed the maximum size of 1 MB

Here we go...

No ideas?

could you go to the firefox menu ≡ > help ? > troubleshooting information, copy the contents of that page and paste that here into a reply on the forum as well? this might give us a clue what is going on...

Application Basics

Name: Firefox Version: 39.0.3 Build ID: 20150806001005 Update Channel: release User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:39.0) Gecko/20100101 Firefox/39.0 Multiprocess Windows: 0/1 (default: false)

Crash Reports for the Last 3 Days

All Crash Reports

Extensions

Name: Adblock Plus Version: 2.6.10 Enabled: true ID: {d10d0bf8-f5b5-c8b4-a8b2-2b9879e08c5d}

Name: DownThemAll! Version: 2.0.18.1-signed Enabled: true ID: {DDC359D1-844A-42a7-9AA1-88A850A938A8}

Name: NoScript Version: 2.6.9.34 Enabled: true ID: {73a6fe31-595d-460b-a920-fcc0f8843232}

Name: Video DownloadHelper Version: 5.3.1.1-signed Enabled: true ID: {b9db16a4-6edc-47ec-a1f4-b86292ed211d}

Name: YouTube™ Flash® Player Version: 1.3.1-signed Enabled: true ID: jid1-HAV2inXAnQPIeA@jetpack

Name: Avast Online Security Version: 10.2.0.187 Enabled: false ID: wrc@avast.com

Graphics

Adapter Description: NVIDIA GeForce GTX 760 Adapter Drivers: nvd3dumx,nvwgf2umx,nvwgf2umx,nvwgf2umx nvd3dum,nvwgf2um,nvwgf2um,nvwgf2um Adapter RAM: 2048 Asynchronous Pan/Zoom: none Device ID: 0x1187 Direct2D Enabled: Blocked for your graphics driver version. DirectWrite Enabled: false (10.0.10240.16390) Driver Date: 7-22-2015 Driver Version: 10.18.13.5362 GPU #2 Active: false GPU Accelerated Windows: 1/1 Direct3D 11 (OMTC) Subsys ID: 28471462 Vendor ID: 0x10de WebGL Renderer: Google Inc. -- ANGLE (NVIDIA GeForce GTX 760 Direct3D11 vs_5_0 ps_5_0) windowLayerManagerRemote: true AzureCanvasBackend: skia AzureContentBackend: cairo 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: 2 browser.download.importedFromSqlite: true browser.download.useDownloadDir: false browser.places.smartBookmarksVersion: 7 browser.sessionstore.upgradeBackup.latestBuildID: 20150806001005 browser.startup.homepage_override.buildID: 20150806001005 browser.startup.homepage_override.mstone: 39.0.3 dom.mozApps.used: true extensions.lastAppVersion: 39.0.3 font.internaluseonly.changed: true gfx.blacklist.direct2d: 3 gfx.blacklist.layers.direct3d11: 3 gfx.direct3d.last_used_feature_level_idx: 0 media.gmp-eme-adobe.lastUpdate: 1437788260 media.gmp-eme-adobe.version: 12 media.gmp-gmpopenh264.lastUpdate: 1431730643 media.gmp-gmpopenh264.version: 1.4 media.gmp-manager.buildID: 20150806001005 media.gmp-manager.lastCheck: 1439324363 media.mediasource.ignore_codecs: false media.mediasource.webm.enabled: true media.webm.intel_decoder.enabled: true network.cookie.prefsMigrated: true network.predictor.cleaned-up: true places.database.lastMaintenance: 1439231044 places.history.expiration.transient_current_max_pages: 104858 plugin.disable_full_page_plugin_for_types: application/pdf plugin.importedState: true privacy.sanitize.migrateFx3Prefs: true storage.vacuum.last.index: 1 storage.vacuum.last.places.sqlite: 1438294086

Important Locked Preferences

JavaScript

Incremental GC: true

Accessibility

Activated: false Prevent Accessibility: 0

Library Versions

NSPR Expected minimum version: 4.10.8 Version in use: 4.10.8

NSS Expected minimum version: 3.19.2 Basic ECC Version in use: 3.19.2 Basic ECC

NSSSMIME Expected minimum version: 3.19.2 Basic ECC Version in use: 3.19.2 Basic ECC

NSSSSL Expected minimum version: 3.19.2 Basic ECC Version in use: 3.19.2 Basic ECC

NSSUTIL Expected minimum version: 3.19.2 Version in use: 3.19.2

Experimental Features

Do you use any video-downloading add-ons? Some might force Flash on YouTube.

Whoops, our posts crossed. What if you disable the "YouTube™ Flash® Player" extension?

Oh dear, having done that I've just noticed I have this Youtube Flash Player add-on. I've disabled it and HTML5 is working. I'm such an idiot. I don't even remember installing that! Thanks for your help all.