Wappalyzer's results for example.com, a 1.3 KB static page with no JS or sub-requests, include eight JS frameworks and two web font providers. Its results for facebook.com include a web framework made by Yahoo and deprecated in 2014. It says Youtube is built on WordPress, and HN is a React app with custom fonts.
I was using the "Technology lookup" box on the homepage, testing with the extension I get only Nginx too. And Youtube seems right and example.com maybe right, though nothing at all for Facebook.
And is available for both Chrome [2] and Firefox [3] as well as a Bookmarklet [4].
The extension that @pier25 is using seems to be available only on Google Chrome [5].
[1] https://www.wappalyzer.com/
[2] https://chrome.google.com/webstore/detail/wappalyzer/gppongm...
[3] https://addons.mozilla.org/firefox/addon/wappalyzer/
[4] https://www.wappalyzer.com/download
[5] https://chrome.google.com/webstore/detail/ui-stack/ijagboogl...