Awesome Screenshot 3.0.21
Firefox 55.0.1
Windows 7 Home Premium x64 SP-1

Takes forever to capture (using this Mozilla page for testing a capture). If I change focus to another window, even just Notepad, the capture is aborted in Firefox. I'd wait for over 12 minutes for a page capture while resisting to change focus to other windows and still the add-on failed: just a constant worthless "Capturing" message. Great, a super-slow add-on that won't continue the capture if focus changes to another window while waiting for their capture to do anything. Other capture programs (e.g., Nimbus Screenshot and Pearl Crescent Page Saver) are immediate when I capture a page (but those others have a short remaining lifetime since they are not Web Extension add-ons). This add-on is unusable (in my setup as noted above).