Some problems with Youtube

[tor000]

Hello:
When I watch a video at Youtube with Pale Moon, I've noticed that after finishing it, when I want to watch it again, pressing the play button, jumps Youtube player from the beginning of the video, to the end of the video, so I had to refresh the video.
Similar thing happens when I watch a video and wants to returns to some earlier part of the video that I want to watch again, and Youtube player jumps from it to the current part that was played.
What causes this?

Application Basics
------------------

Name: Pale Moon
Version: 27.4.1
Build ID: 20170801005452
Update Channel: default
User Agent: Mozilla/5.0 (Windows NT 6.1; rv:45.9) Gecko/20100101 Goanna/3.2 Firefox/45.9 PaleMoon/27.4.1
OS: Windows_NT 6.1
Multiprocess Windows: 0/8 (default: false)
Safe Mode: false

Extensions
----------

Name: Download YouTube Videos as MP4
Version: 1.8.10
Enabled: true
ID: {b9bfaf1c-a63f-47cd-8b9a-29526ced9060}

Name: IE View
Version: 1.5.6.1-signed.1-signed
Enabled: true
ID: {6e84150a-d526-41f1-a480-a67d3fed910d}

Name: NoScript
Version: 5.0.7.1
Enabled: true
ID: {73a6fe31-595d-460b-a920-fcc0f8843232}

Name: Session Manager
Version: 0.8.1.13
Enabled: true
ID: {1280606b-2510-4fe0-97ef-9b5a22eafe30}

Name: uBlock Origin
Version: 1.13.8
Enabled: true
ID: uBlock0@raymondhill.net

Graphics
--------

Adapter Description: NVIDIA GeForce GT 220
Adapter Drivers: nvd3dum nvwgf2um,nvwgf2um
Adapter RAM: 1024
ClearType Parameters: Gamma: 2200 Pixel Structure: R ClearType Level: 100 Enhanced Contrast: 50
Device ID: 0x0a20
Direct2D Enabled: true
DirectWrite Enabled: true (6.2.9200.22164)
Driver Date: 11-14-2016
Driver Version: 21.21.13.4201
GPU #2 Active: false
GPU Accelerated Windows: 8/8 Direct3D 11 (OMTC)
Subsys ID: 040110b0
Vendor ID: 0x10de
WebGL Renderer: Google Inc. -- ANGLE (NVIDIA GeForce GT 220 Direct3D11 vs_4_1 ps_4_1)
windowLayerManagerRemote: true
AzureCanvasBackend: direct2d 1.1
AzureContentBackend: direct2d 1.1
AzureFallbackCanvasBackend: cairo
AzureSkiaAccelerated: 0

Important Modified Preferences
------------------------------

accessibility.browsewithcaret_shortcut.enabled: false
accessibility.typeaheadfind: true
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.frecency_experiment: 3
browser.download.importedFromSqlite: true
browser.link.open_newwindow: 2
browser.places.smartBookmarksVersion: 8
browser.search.context.loadInBackground: false
browser.search.useDBForOrder: true
browser.sessionstore.max_tabs_undo: 99
browser.sessionstore.max_windows_undo: 99
browser.sessionstore.upgradeBackup.latestBuildID: 20170628075643
browser.startup.homepage: google.co.il
browser.startup.homepage_override.buildID: 20170801005452
browser.startup.homepage_override.mstone: 3.2.2
browser.tabs.drawInTitlebar: false
browser.tabs.insertRelatedAfterCurrent: true
browser.tabs.loadInBackground: false
browser.tabs.remote.autostart.2: true
browser.urlbar.searchSuggestionsChoice: true
browser.urlbar.suggest.searches: true
browser.urlbar.trimURLs: true
dom.max_chrome_script_run_time: 40
dom.max_script_run_time: 40
extensions.lastAppVersion: 27.4.1
font.name.serif.x-western: Arial
gfx.crash-guard.d3d11layers.appVersion: 54.0.1
gfx.crash-guard.d3d11layers.deviceID: 0x0a20
gfx.crash-guard.d3d11layers.driverVersion: 21.21.13.4201
gfx.crash-guard.d3d11layers.feature-d2d: true
gfx.crash-guard.d3d11layers.feature-d3d11: true
gfx.crash-guard.status.d3d11layers: 2
gfx.crash-guard.status.d3d9video: 2
gfx.direct3d.last_used_feature_level_idx: 0
image.http.accept: image/webp,image/jxr,image/png,image/*;q=0.8,*/*;q=0.5
media.benchmark.vp9.fps: 62
media.benchmark.vp9.versioncheck: 2
media.gmp-gmpopenh264.abi: x86-msvc-x86
media.gmp-gmpopenh264.lastUpdate: 1499166377
media.gmp-gmpopenh264.version: 1.6
media.gmp-manager.buildID: 20170628075643
media.gmp-manager.lastCheck: 1501062529
media.gmp-widevinecdm.abi: x86-msvc-x86
media.gmp-widevinecdm.lastUpdate: 1499166380
media.gmp-widevinecdm.version: 1.4.8.903
media.gmp.storage.version.observed: 1
media.hardware-video-decoding.failed: false
media.mediasource.format-reader: false
media.mediasource.webm.enabled: true
media.peerconnection.ice.no_host: true
media.webrtc.debug.log_file: C:\Users\Doron\AppData\Local\Temp\WebRTC.log
network.cookie.cookieBehavior: 1
network.cookie.lifetimePolicy: 2
network.cookie.prefsMigrated: true
network.http.speculative-parallel-limit: 0
network.predictor.cleaned-up: true
places.database.lastMaintenance: 1502723741
places.history.enabled: false
places.history.expiration.transient_current_max_pages: 80492
plugin.disable_full_page_plugin_for_types: application/pdf
plugin.importedState: true
plugin.state.flash: 1
plugin.state.java: 0
plugin.state.npauthz: 0
plugin.state.npdeployjava: 0
plugin.state.npspwrap: 0
privacy.cpd.cookies: false
privacy.history.custom: true
privacy.sanitize.migrateFx3Prefs: true
privacy.sanitize.sanitizeOnShutdown: true
security.sandbox.content.tempDirSuffix: {74972040-cdfa-44e4-937a-9b3f8aa9141f}
storage.vacuum.last.index: 1
storage.vacuum.last.places.sqlite: 1502723741

Important Locked Preferences
----------------------------

Places Database
---------------

JavaScript
----------

Incremental GC: true

Accessibility
-------------

Activated: false
Prevent Accessibility: 1

Library Versions
----------------

NSPR
Expected minimum version: 4.13.1
Version in use: 4.13.1

NSS
Expected minimum version: 3.28.5.1
Version in use: 3.28.5.1

NSSSMIME
Expected minimum version: 3.28.5.1
Version in use: 3.28.5.1

NSSSSL
Expected minimum version: 3.28.5.1
Version in use: 3.28.5.1

NSSUTIL
Expected minimum version: 3.28.5.1
Version in use: 3.28.5.1

Experimental Features
---------------------

[Moonchild]

It's likely caused by the fact that you've enabled the old format reader and webm. There are known issues with the old reader because it draws on an older version of the MSE spec.

[tor000]

Here is a screen capture of my Pale Moon video settings:

[trava90]

There are known issues with MSE + WebM which are being worked on. For now it is highly recommended that you disable MSE + WebM and use MSE asyncronously (which uses a newer version of the MSE spec and is more compatible with the current web).