Firefox default tool refuses to take lossless screenshot of long pages (.png or lossless copy). Sounds crazy, but it's true - you can read the issue "Ignore the PNG to JPEG Cutoff Limit...". This addon can be used as a workaround. Another workaround is to use developer tools screenshot (devtools.command-button-screenshot.enabled in about:config).

Format string similar to default tool is "Screenshot_%Y-%m-%d %t".