Расширение Nimbus Screen Capture оказалось одним из наиболее удобных среди аналогичных:)

It Does The job, i like it

I just love Nimbus!

GREAT! I love this add-on. It gives Firefox some extremely useful functions.

Everything works well, but copying to clipboard does not work for me

Excellent utility that routinely just doesn't work at all unfortunately. I use this a LOT for my job and would be willing to pay if it was more stable. It seems like every 48hrs it simply does nothing when clicked (well the UI shows asking you what sort of capture to do) and then no capture occurs. Sometimes restarting the browser fixes it.

It works like a charm!