Strange Cyrillic Pop Up on YouTube (only when using Firefox)

[SusiSu]

Nevermind, I've found the culprit. Suddenly thought of the extensions I have loaded on and it was one of those.  I've removed it.  No more pop up.

Does anyone else get a strange orange pop up, with Cyrillic writing, when visiting YouTube using Firefox?  I've been getting it for a couple of weeks now.  I click into YouTube using my bookmark (I have also tried using a search engine and following that link.  It makes no difference) and I immediately see an orange rectangle blocking the screen with what looks to me like Cyrillic (Russian?) text.  I'll shut it down and get the same thing the second time, but not on further attempts.

I've scanned my computer.  I have no issues.  This only happens on Firefox and only on YouTube.  If I open YouTube in Chrome, Brave or DuckDuck I don't see it.  Similarly, I don't see it on any other website when I'm using Firefox.

Any ideas?

[wjmann]

What is the Name of the Extension?  I have this showing up too.  Since I added quite a few extensions, it'll be faster if you mention it to save me some time and concern.  I only experienced it on Firefox with Youtube (even within the privacy window).  Because I can't read whatever cyrillic language its in, it left me more than a little troubled. You can e-mail me if you want to keep it private.

[TLBoland]

Same thing here. It's only when I enter YouTube, and my default browser is Firefox. Likewise, I've scanned my system repeatedly, attempting to 'catch it' with various programs running, but the scans all come back with zero detections and no further course of action. I am assuming the op-up isn't malicious or destructive, just very annoying, and I want it to be gone. I've added so many extensions trying to prevent pop-ups and ads, I don't want to have to start over from scratch, so identifying the offending extension would be greatly beneficial. Thanks in advance.

[SusiSu]

Sorry I should have thought to say which one it was.  It's called "SaveFrom.net helper all-in-one/YouTube downloader". I've attached how it looks on the Extensions page.

The reviews on it mention the pop up.  Apparently it is prompting payment for a full version of the extension.