I've been a heavy user of Marker on Chrome for a while, but I'm super happy that it's now available on Firefox, as it is my daily browser.
The fact that Marker lets you capture, annotate and then share your screenshots exactly the way you want makes it a must-have if you're involved in the process of creating or testing any type of web site or application.

Absolutelty AMAZING! I use the Marker.io Firefox extension to debug my website across all browsers. Whenever I find a bug on my website, I grab a screenshot with the extension, add annotation and create an issue in JIRA, a card in Trello or a message in Slack if our team still need to discuss the bug.

Highly recommended!