Bug 209907

The webRTC API offers mechanism to generate, preserve and reuse certificates, allowing users to verify 
the identity of a peer without consulting on the webserver. 

The API lets a page generate a certificate with a validity of up to one year, then store it in IndexDB.
Subsequent page loads can read the certificate from IndexDB and use it as part of an RTCPeerConnection.

https://www.w3.org/TR/webrtc/#dom-rtcpeerconnection-generatecertificate

Notice that the generated certificate is opaque - it cannot be exported/backed-up/restored from Javascript.

The proposed change in ITP will (If I understand it right) erase IndexDB after 7 days. 
This would make the generateCertificate() almost useless.

Here is an example of how we (https://pi.pe) use this feature:

My heating controller trusts webRTC connections from my ipad because it checks that the certificate offered by my ipad during DTLS setup is in the heating controller's trust store. This allows the 2 to communicate securely with no MiTM and no need for a centralized authority.

With your proposed change for this to work, I'd need to be _sure_ that I'd visited the heating management site at least once a week.
My current visit pattern is rarely during the summer months and every few weeks during the winter.

I'd add that the W3c's Proposed IDP identity validation scheme for webrtc also depends on persistent certificates.

Other use cases include app-free access to security cameras, baby monitors et al.

(ref: https://webkit.org/blog/10218/full-third-party-cookie-blocking-and-more/ )