The only thing that I don't do in Firefox's "Private Browsing" mode is play a handful of stupid little in-browser games that save progress in a cookie or whatever. I even have Firefox set up to open in "Private Browsing" by default. Here's what I did just now:

1) Quit Firefox

2) Opened Firefox

3) Visited 'about:config'

4) Set 'javascript.enabled' to 'false'

5) Quit Firefox

6) Opened Firefox

7) Re-visited 'about:config' and verified that 'javascript.enabled' is still set to 'false'

8) Visited <https://datatracker.ietf.org/doc/rfc5737/>

It's still exactly like I reported it was. The "Manage browsing data" thing accessed through Firefox's regular settings dialog doesn't indicate that there is any data saved by any ietf.org subdomain, and when I watch the Network pane, a ctrl+shift+f5 reload of the RFC5737 page indicates that the page loads everything from an ietf.org subdomain... so the saved resources from one of the like eight domains in that list aren't relevant.