I tested Version 3.0.21 (Last Updated: June 22, 2017)
on Firefox 54.0.1 (32 bit) on Windows 8 with 3.5 GB memory.
It installs Ok.
The first page I tested it on was a small reddit subreddit.
It captured the page and opened a save page dialog to save the capture as a PNG file. This worked.

The second page I tested it on was 4chan's Comic and cartoon catalog, not particularly large with small thumbnails.

This also worked, the webpage is longer at 11000 pixels and I noticed the capture took longer.

Note it captures any hover regions as you would see them yourself when scrolling down a page.

I tested it on four more pages, largest size 2.8 MB, smallest 53 KB.

I've been a long time user of the older Pearl Crescent Page Saver, and was looking for a new saver for my new firefox profiles, and this fits the bill.

EDIT: I further tested changing the File name pattern to "%t %Y-%m-%d-%H-%M-%S" - to get finer-grained timestamps - and this worked.

* can take visible portion
* can take full page
* full page is real full page even if the page has a header area that stays if page is scrolled - many other screenshot tools cannot handle that
* has options to configure

- cannot select regions to be captured (please add this feature)
- no keyboard shortcuts
- page needs to be scrolled (do tue web extension?)

- missing option to capture only a region