Why does Firefox blur the webpage when the Esc key is pressed three times?

Question

On my Dell Latitude 3120 2-in-1 laptop Firefox blurs the webpage (and sometimes destroys the site layout doing so) for a few seconds when I press Esc three times in a row and
closes the current tab and replaces it with a tab in my default search engine when I press Esc four times in a row.
Has anyone an idea on why this happens?
By the way: it happens both on Windows and on Linux, both with the standard installation of Firefox. On Firefox-built-in sites it doesn't work.

Answer 1

Ok, I found the problem myself - it was a privacy function of the "Startpage.com" extension I had installed. It blurs the page so you can minimize the window and nothing important can be seen in the preview and one Esc click more it and closes the current tab and opens Startpage.com's search engine.
How I found out? I uninstalled the extension because it didn't really help me and I was decluttering my browser and suddenly the behaviour stopped.
And it didn't work on Firefox-built-in sites because extensions don't work on such sites.