Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

Thanks Fred. Safe Mode doesn't solve the problem. Tracking protection is disabled, despite the fact exeption for YT is also established. Only deleting site specific configuration data solves the problem, but it's only momentary, until reloading the tab or open a new one.

Thanks Fred. Safe Mode doesn't solve the problem. Tracking protection is disabled, despite the fact exeption for YT is also established. Only deleting site specific configuration data solves the problem, but it's only momentary, until reloading the tab or open a new one.

Do the thumbnails work in private mode?

Thanks Blurredsight.

1)- Start FF in non-Safe Mode. 2)- Load YT. Thumbnails don't work. 3)- Restar in Safe Mode, Private window, or not, doesn't matter anymore, thumbs don't appear. 4)- Delete site-specific conf data. Thumbnails start to work in all windows and tabs you have open at that time. 5)- Close all windows, restart FF, Safe Mode or not, Private or not, browse the web WITHOUT load YT. 6)- Restart FF in Safe Mode. YT thumbs will continue to work in all windows and tabs.

RESULTS: Once you loaded YT in NON-SAFE Mode, thereafter thumbs won't work, no matter you restart now in Safe Mode, or Private, until you delete site-specific conf data. Once you deleted site-specific conf data, thumbs will continue working only in Safe Mode, Private or not, until you load YT in non-Safe Mode, and thereafter it will stop work again.

Update: All said in previous post is valid disabling AdBlockPlus completely. No need to Safe Mode. I need yet to delete site-specific configuration after disabling ABP. It's not enough to disable it in a per-site (YT) basis, as I did until now. It's needed to disable ADB completely. Thanks for reading.

Load the web page. After the page is loaded, click the AdBlock icon. Now select Disable on <website>. Reload the page.

Sorry, all my tests were done with ABP disabled on YouTube

Hi ogalati, glad to see you on Mozilla Support Forum.

Try Refreshing your browser:-

• Click the menu icon, then click on “Help”. Click on Firefox's main menu button, represented by three horizontal lines.
• Click “Troubleshooting Information”.
• Click on “Refresh Firefox”.
• Confirm.
• Click “Finish”.

If it doesn't solved your problem, then feel free to ask in Reply Section. Thanks for raising your question in Mozilla Support Forum.