I just tested about 6 or 7 different screenshot extensions — including the big ones like Nimbus, Awesome Screenshot, and Fireshot — and Save Screenshot by Manuel Reimer was the only one that could actually (and COMPLETELY capture and) save the huge webpages I needed to screenshot for archiving purposes. Not only was the capturing process itself much faster the others I have tested in the past, it captured the entirety of the first webpage (in one try) as a 26.5 MB PNG file without any unnecessary loading, superfluous options, or other fluff that normally gets in the way of performing what should be a simple task in a timely manner. It delivered and did everything it promised to do for me as of November 8th, 2018 on my Firefox 62.0 Portable browser — in private browsing mode no less with a VPN on without needing to tweak anything beforehand.

*Funny story about this extension:

I had been so frustrated with the repeated failures of the other webpage capture extensions that, when I didn't see an immediate reaction to hitting the screencapture button, I assumed it wasn't working like all the others and uninstalled it immediately — within seconds because I had my extensions tab open for quick access. To my dismay the "save file as" window popped up almost immediately after I had uninstalled it. I was still able to save the image despite the extension being uninstalled and immediately undid the uninstallation once I realized it had worked. Talk about a close call.