Firbolgs get the Firbolg Magic trait (VGtM, p. 107), which lets them cast Detect Magic. Normally, Detect Magic is able to be cast as a ritual.
Can this version be cast by a Druid as a ritual, or can it only be cast normally as a free once-a-day spell?
Best Answer
Your Firbolg Druid cannot cast their racial Detect Magic as a ritual
The rules for ritual casting are given in the Spellcasting chapter of the basic rules:
The key paragraph is the second one, laying out the requirements to be able to cast a given spell as a ritual:
Firbolgs do not inherently have any trait which allows ritual casting, so they cannot cast any rituals by default. But your Firbolg is a druid with the druid's Spellcasting feature - which allows them to cast certain rituals:
Unfortunately this doesn't allow them to cast Detect Magic as a ritual, because the Firbolg Magic trait says:
... but it doesn't state that you have them prepared in a spellcasting sense - you simply have a special ability which allows you to produce their effects once per rest. Since the spell is not prepared, it doesn't qualify for use with the druid's ritual spellcasting - to be able to cast it as a ritual, you'd have to prepare it as a druid spell and use up the space on your prepared spells list.