Call the local fire department non-emergency number and ask if they can schedule a visit to inspect your fire alarms and provide recommendations on the situation. The fire department is genuinely interested in your safety, because it's also important for their safety so they don't have to come rescue you. If anything is a fire hazard, the professionals can explain why and explain how to fix it. But they'll probably say "WTF" because the landlord is most likely just being a fuckface, as landlords do. Assuming the latter, ask them if you get the "WTF" in writing so you can wave it in the landlord's face when you tell them to fuck off and die.

I've never heard of the quantity of furniture being a fire hazard unless it impeeds the ability to exit during a fire. Is it made out of dry tinder and balloons of gasoline?

Ideally I’d like to help them relax and ensure them it’s not something they can get evicted over.

it is something they can get evicted over- or potentially. That very much is determined by lease they signed and the building codes where you're at.

And yes, it is fairly subjective. Generally speaking, clear access to the points of egress is a must- this includes bedroom windows. (though it's normal to push a bed up against them.) You'd also probably want to consider getting EMS/Fire in there for a medical emergency (these kinds of building codes are usually lumped in with fire code, in large part because it's common for firemen to respond to medicals as well.) They'll likely be very grumbly if they can't at least bring in the board (the rigid board they use to get people onto the wheeled EMS cart/bed.) to get them on. this means enough space for 2-3 "well-marbled beefcakes" to stand around a guy and the board (which is about the size of a guy.)

fire hazard die to the amount of furniture

Never heard such a bull*

Do not sign anything.

Let them talk, do your laughing later.