[Firefox] Right-mouse click produces context menu

[Machariel]

See screenshot in the attachment.
When I click the right mouse button on chess.com on the board I get a context menu. It's not supposed to be. I wonder if it's a add-on or something. I disabled them all but the context menu still appears where it shouldn't.

 

[wither 2]

That's the standard context menu in Firefox when you right click on any webpage.

You can try the suggestion here to lose it-

context menu - How to deactivate firefox right-click when I am on a Google Drive document? - Super User