What are you seeing with DNS? Does it matter whether your Firefox is configured for DNS over HTTPS or not? See: Firefox DNS over HTTPS.

For WebRTC leaks, you can modify a setting. I haven't looked at this for ages, but here's an older post and you can test whether it still works: https://support.mozilla.org/questions/1303685#answer-1348340

I'm using a very well supported VPN (Surfshark); to eliminate IP leaks I had to go into the sysctl.conf file & add 4 lines to essentially disable IPv6 from leaking. The DNS issue was temporarily solved by changing to Python-based browser. I will run through you suggestions tomorrow & give you feedback as to what occured before calling it solved.-DG

These are the links where I test for IP & DNS leaks: https://surfshark.com/dns-leak-test https://surfshark.com/what-is-my-ip

I ran thru your 2 links using only https:// settings in Firefox & the "about:config" changes. When I go to the above 2 links, leaks still show.

Powering down, then back into Linux Mint 20, I found in my "not big 3" browser that DNS leaks checks passed. Now, though, IP leaks failed in the IPv6 area. Back to VPN support.

I wonder whether some connection settings need to be adjusted in Firefox? In the same dialog where you choose DNS over HTTPS (or not, in your case), there are proxy settings, in case those are relevant to using SurfShark.