They can, it just takes extra steps.

Firefox tries DoH via Cloudflare, for an internal domain that returns NXDOMAIN (Cloudflare can't answer for your internal resolver,) then they fall back to local resolvers, which is OS based (DHCP or statically set.)

The response time to complete the internal request goes up, because you're sending data to Cloudflare, they can't find it, then the 'normal' response time for internal resolvers.

Edit: Made more clear.