How did you uninstall it? Can you check the Add-ons page to confirm it was removed? Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. On the right side, find the "Manage Your Extensions" heading.

If you see the extension here, click its ... menu button and use Remove.

Otherwise...

It's possible that the extension modified your YouTube cookies to take advantage of particular features of the YouTube site. To clear that up, you could try:

(1) Clear Firefox's Cache

The goal is to remove previously served files that might be customizing your experience. See: How to clear the Firefox cache (just cached web content, not all site data)

If you have a large hard drive, this might take a few minutes.

(2) Remove your YouTube cookies (save any pending work first). While viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that.

In the dialog that opens, you will see one or more matches to the current address so you can remove the site's cookies individually without affecting other sites.

Then try reloading the page. Does that help?

I uninstalled the add-on in "Add-on Management". After clearing cache and the youtube cookies, nothing has changed either. This is strange.

It's working! I found the information I needed on the official Fire Fox website. Thanks for your help.