The magic most likely protects the occupants
Bringing real-world physics into the discussion of magic is always risky, and your quote above specifically tells you about what's happening inside the hut: it's comfortable and dry...because magic. The likely intent of Tiny Hut is to create a safe place to spend 8 hours.
As Speedkat says below in comments, if we go by the adage of spells do what they say and no more, then there are no circumstances in which the atmosphere is anything but comfortable and dry because the spell doesn't state that there are.
The idea of ways to make it unsafe by introducing real-world physics into the equation is risky and makes the magical spell seem less powerful. This isn't a physical structure that's created that works within the physical world, it's magic. Let the magic be magical.
This doesn't mean that a DM may rule otherwise, but in general that line states that the interior is a safe place to be no matter what's happening outside of it.
In addition, the easiest counter to Tiny Hut is simply having someone able to come in to Dispel Magic on it.
Commentary: Does this make Leomund's Tiny Hut overpowered?
I do not think it does, mostly because it is very situational. In a typical combat, having at least a minute to cast the spell is highly unlikely. In a scenario where you've got time to prepare, the enemy does as well and there are several obvious ways to counter the tiny hut from a simple cast of dispel magic to treating the hut as a fortress under siege to simply ignoring it and waiting out the time. If your enemy is all camped in one location, there may be multiple other targets that are now vulnerable. Don't take the situational factor of when Tiny Hut does it's job as judgement that it's overpowered.
You Cannot Teleport Into the Tiny Hut
A key, though subtle point to this opinion is a statement on Range in the PHB (203)
Once a spell is cast, its effects aren’t limited by its range, unless the spell’s description says otherwise.
The suggestion is that during and before spell casting the spell's effects are limited by range.
Jeremy Crawford weighs in on this issue here, specifically in this tweet:
The indigo layer of prismatic wall prevents you from casting spells through it. This doesn't stop a spell with a range of self, such as misty step, but it does stop dimension door, which has a range of 500 ft.
and Leomund's Tiny Hut (PHB 254):
Spells and other magical effects can’t extend through the dome or be cast through it.
Because of it's range of 500 ft, casting Dimension Door to teleport into the Tiny Hut would involve casting the spell through the dome and cannot be done.
Best Answer
The hut keeps out weather, magical effects, creatures and objects, but ask your DM
Tiny Hut says:
The statement of comfortable and try atmosphere is only in relation to the weather outside. The hut does not create such an atmosphere against other sources, and it does not create breathable air.
Object is a defined game term and under this definition poison gas or heat is not an object.
Therefore, natural poison gas, or heat from a natural fire will affect the inhabitants of the hut, just like poison gas or heat from dragon breath. Technically, fire or poison gas from dragon breath is no different than any other mundane fire or poison gas. As will any other non-magical effect that is neither a creature, object, nor caused by the weather.
(There may be a fringe case for alien environments in the Abyss or such, where poison gases occur as part of the weather. Those should be kept out).
Because tiny hut is an ritual, it is a really good spell — it does not cost you a slot, and it does cost no preparation. I track how often I cast spells with my wizard, and it is one of my most often used spells. It still is good, even if it allows smoke or heat to pass. Firstly, the monsters outside might not know the spell, and might not know smoke would pass through. Secondly, you can shoot out with arrows, so setting up fire nearby is perilous.
That said, check with your DM. Even though the spell does not simply say the atmosphere is fine, full stop, the normal use of the sphere is to enable safe resting. If someone can smoke you out that makes it a less useful spell. The DM may be fine with ruling the atmosphere hedges out normal gas or fire, where it does not for the poison gas or fire from dragon breath. Or even from dragon breath if its not "special".