It was created as part of a solution to the infamous browser crypto chicken-and-egg problem, and as part of a larger effort to make browser-crypto applications more secure.
Page Integrity is available from addons.mozilla.org.
After installing the Page Integrity extension, you’ll notice the PI button in your browser toolbar. Click the button to view integrity information about the currently loaded page in the active tab of your browser, including the SHA-256 checksum hash of the page source. Additionally, if the page source is signed with one or more digital signatures, PI will verify the signatures, and show the signer's public signing keys for these signatures.
For more information about the 'browser crypto chicken and egg problem', using Page Integrity to verify the integrity of web pages, and signing web pages for verification using Page Integrity, please see https://www.pageintegrity.net/.