I've seen something like that before. The browser stops responding correctly, and black rectangles appear on whatever page is open and on the browser toolbars after I close toolbar menus or click another tab. It's as if the whole browser stops replacing the graphics of the things behind the things I close. I always thought it was a memory or CPU issue. I think it happens on sites that have many entries in NoScript. Other people have reported it in Firefox, Chrome, Edge. Most answers say to disable Hardware Acceleration or GPU. I still think certain heavy webpages are the cause. If I see more, I'll save them.

https://support.mozilla.org/en-US/questions/1006033
https://support.mozilla.org/en-US/questions/925894
https://www.reddit.com/r/firefox/comments/3cl8kk/firefox_39_black_recta…

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