What the H is this?

At a couple forums i started seeing this rectangular box surrounding any test my cursor is on and it has the little message box as in the pic below. The text of the message varies depending on the text, or the element. It seems to be describing the java script or html element or some such thing. It also stops my typing freezing my cursor at times for minutes till i move the cursor away and click elsewhere. Thats my concern. Then i can usualy go back and type again. Is this some sort of toolbar affair or virus that installed itself or a browser dev function that somehow got turned on or what? Driving me nuts !

EDIT: i also just noticed it in my webmail. Theres a line across the screen and a perforated line above that.

Pale moon, and yeah, i think thats what it was but after a restart it seem to be gone. I likely accidentally pressed some key combo that started it.