Redact v1.0.0
Redact is a small Firefox extension that redacts text on a page so you can screenshot it without having to edit the image afterwards.
To use it, just highlight some text and choose "Redact" from the right-click menu. To get rid of the redactions, reload the page.
That's all there is to it! Redact stays out of your way.
Note: This doesn't actually remove the text from the page, just blacks it out – so it is not suitable for printing to PDFs and the like, just for screenshots. (This will probably be fixed in the future.)