Firefox has some weird slowness with DNS that I have troubleshot to death. I still use it for almost everything, but sometimes I'll have an entire day of 30s page loading times.
I put this setting in ages ago on my FF profile and haven't seen DNS lag.
My biggest DNS lag was before I used PiHole and was relying on my router, which upstream to 8.8.8.8. I've just assumed that little thing was overloaded or that Comcast was just having a "hiccup".
This is almost certainly a fragmentation issue caused by lower MTU and broken path MTU on the VPN. Drop the system to 1280 to troubleshoot, if things work immediately there's the culprit, raise it up til it doesn't or don't, I keep my VPN's at 1280.
EDIT: I do not know why its an issue with firefox and not chrome, it's likely QUIC fucking up since it cant fragment and needs to fall back to TCP, chrome is probably error handling this better... dropping the MTU that low will make the fallback explicit: https://blog.apnic.net/2019/03/04/a-quick-look-at-quic/
Same here. Tends to be pretty inconsistent. DNS-over-HTTP(s) definitely disabled. 30s is a lot more than I've experienced, but there are times where it clearly struggles to look things up.
