Fixing slow Firefox loading when using Pi-Hole

I run Pi-Hole to prevent clients on my network from loading dangerous or gross things like advertisements or tracking scripts. This normally works great: it functions on all devices without requiring configuration and I don’t need to think about it too often.

I’ve also been using Firefox a lot more often lately, partly because Big Sur introduced a lot of Safari regressions and sites like Twitter are broken regularly, and partly because I like the features it’s coming out with like HTTPS-only mode.

Unfortunately, Firefox and Pi-Hole do not play nicely together when it comes to certain websites. For example, when loading several of its trackers have performance issues:

A graph of timings for a network request. The first is 'Blocked' with 5.11 seconds. After this, staggered after Blocked, is 'DNS Resolution' which shows a 1.01 second timing. The rest are 0 seconds: connecting, TLS setup, sending, waiting and receiving.

When Firefox comes across a host which resolves to, it appears to have some kind of internal retry mechanism that, combined with HTML’s sequential loading of scripts, causes a cascading set of delays making loading take an extremely long time.

Fortunately, Pi-Hole’s behavior of returning for disallowed hosts is configurable. Changing its BLOCKINGMODE to NODATA changes the resolution behavior from:

$ dig +noall +question +answer
;		IN	A	2	IN	A


$ dig +noall +question +answer
;		IN	A

Instead of providing an IP address, the response we get is instead that there are no A records for the domain, and Firefox gives up a lot faster, taking a few milliseconds instead of a few seconds. The Pi-Hole documentation on blocking modes provides a caveat:

…experiments suggest that clients may try to resolve blocked domains more often compared to NULL blocking…

The default blocking behavior (NULL) is returning I have not come across any issues with this change, but I also don’t think I’d notice if DNS requests drastically increased on my network.

This doesn’t resolve all of the performance issues on the SF Chronicle in Firefox. Even using a non-Pi-Hole DNS server shows significant loading delays compared to Safari. This, at least, makes it painful instead of frustrating. As an aside, I am resentful that I’m paying $12 per month for a website that wants to inject the scummiest of Taboola-level ads on me.