Firefox's feature of sending DNS lookups over HTTPS improves user security by preventing eavesdropping on DNS queries. To do this Firefox sends all DNS requests to Cloudflare or another configurable server that supports DoH. These servers don't know about systems that are only reachable internally at Dartmouth. Two options to keep resolving Dartmouth-internal DNS names are to disable DoH globally for Firefox or to add dartmouth.edu to the list of domains where DoH is bypassed. Once either of these options are used, you'll be able to resolved Dartmouth internal servers again.