While Safari for Mac doesn't include an equivalent feature, fortunately Firefox does, so if you want to capture full webpages in macOS, you can do so easily by following these steps.
- Download and install Firefox from the Mozilla website (it's free).
- Launch Firefox on your Mac and visit the website from which you want to capture a full webpage as a screenshot.
- Scroll down to the bottom of the webpage in question so that any images set to lazy-load are displayed in full.
- Now right-click (Ctrl-click) anywhere on the webpage and select Take Screenshot from the contextual menu.
- To capture the entire webpage, click Save full page in the top-right corner of the browser window.
- In the framed capture that appears, click either Copy to copy the image to your clipboard, or click Download to save the image PNG file to your hard drive. The file will be saved to the same folder you use as Firefox's default download location.
Note that in step 4, you can drag or click on the page to select a region and then save just that region by clicking Download, or you can save just the visible area of the page by clicking Save visible in the top-right corner.
Tip: You can also customize the Firefox toolbar to add a Screenshot button. Click the Menu button (three lines) in the top-right corner of the browser window. A special tab will open, where you can drag and drop items in or out of the overflow menu and the toolbar. Simply drag the Screenshot item (the cutting scissors icon) to the toolbar, then click Done.