[SOLVED] Looking for help getting my VPN to work with Firefox privacy settings

I recently moved to a new place with a new ISP, and my Mullvad VPN isn't playing nicely with Firefox like it used to. Can any of you networking gurus please help me troubleshoot?

When the VPN is enabled, most requests from the browser fail immediately. If I pull up the dev tools Network tab, I can see that these requests fail with an NS_ERROR_FAILURE message before any data is transferred.

I have Firefox configured to use "strict" Enhanced Tracking Protection. When I reduce it to "standard" my requests go through.

I'm also trying to use DNS over HTTPS with a custom provider (Mullvad, via https://dns.mullvad.net/dns-query). I'm configuring this in Firefox, using the "Increased Protection" DoH setting. When I do that, Firefox reports the DoH status as "Status: Not active (NS_ERROR_FAILURE)". This happens even when Enhanced Tracking Protection is set to "standard" — in other words, that reduced setting fixed the NS_ERROR_FAILURE for HTTP requests, but not for DoH.

So how do I fix this so Strict Enhanced Tracking Protection, DNS over HTTPS, and Mullvad all work together? I never had this problem with my old ISP, so I suspect something's being blocked at the WAN level that I need to circumvent.

• OS: macOS Sonoma 14.5
• VPN protocol: WireGuard
• ISP: AT&T Fiber

I'm just using the official Mullvad client app with mostly default settings. The fiber gateway modem/router came with some default packet filtering firewall rules but I disabled everything in the admin panel. Weirdly, rebooting my machine fixed this temporarily, but the next time I disconnected/reconnected the VPN it broke again. Other browsers (with default settings and no DoH) are working fine when the VPN is connected.

Edit: Solved! Solution here.