Firefox can capture a whole scrolling page without installing anything. Here are the three built-in ways, what each is good for, and what to do when the page is too long or contains things you shouldn't share.
It saves a PNG. Good for most ordinary pages.
Open the Web Console (Ctrl+Shift+K, Mac Cmd+Option+K) and type:
:screenshot --fullpage
Useful options: --clipboard copies instead of downloading, --delay 3 waits three seconds (for hover menus), and --dpr 2 captures at double resolution.
Press Ctrl+P, set the destination to Save to PDF, and save. You get every part of the page as a multi-page PDF, but in the page's print layout — menus, backgrounds and some images may be hidden or rearranged. More on this: how to save a web page as a PDF.
Saves the whole page as PNG, JPEG or PDF in one click — or just a region you drag, or a single scrolling panel (an inbox, a chat, a dashboard pane) that Firefox's own tool can't reach. Before capturing, you can drag boxes over anything private and it is painted out before the image is created, so an unredacted copy never exists. Nothing is uploaded; there's no account and no tracking.
The Firefox version is in review at Firefox Add-ons; a link will appear here once Mozilla approves it.
Get it for ChromeUsing Chrome? See how to screenshot an entire webpage in Chrome, or scrolling screenshots on any device.
More on this topic: How to take a full page screenshot in Microsoft Edge · Redact an image · Redact a PDF · How to take a scrolling screenshot on a Chromebook