@GoldCashew
Is the belief of a Firmement a mainstream belief within TFES, a kind of 50/50 split belief within TFES, or a fringe belief within TFES?
Personally, I eschew belief from knowledge/fact, especially scientific.
I don't know the tfes "stats", but it has been my experience that many, possibly even most, believe, speculate, and/or deduce the existence of a dome. There is also historical, cartographical, and mythological support for its reality.
what are the specifics behind its composition, material properties, and thickness. I have seen one FE'er describe the material as a glass dome, but not sure if that's the generally accepted belief.
No one knows. A major source for the descriptions you seek is the bible, which describes it as a "dark crystal" of some sort of terrible hue. It is also described as "tent like".
Some say that disney (their logo/intro silhouette - supposedly depicting the tower of babel peeking through the firmament) and the modern/recent atlantis conceptions are more appropriate/conceivable.
Others talk about skystone - a mysterious and entrancing blue oxygen laden mineral that is very odd indeed. The locals attributed to the first find of it supposedly claimed the stones were part of a fallen city that floated above which fell due to hubris. All speculation of course.
How thick would such a Firmamemt need to be?
How much wood could a wood chuck chuck? There are too many unknowns and unknowables to justify bothering to calculate it at all.
How is the firmament dome connected to the flat Earth plane?
The bible says it is on pillars, possibly 4 of them - corresponding to the 4 corners. No one knows for sure - we need more data!
Have Firmaments been observed on other celestial bodies?
Not to my knowledge. There is a logical error in thinking the lights in the sky have any relevance to the shape or composition of the earth. If you want to know something about the earth, you have to study IT! Looking the literal opposite direction is unscientific and foolish.
@Longitube & others
The density gradient of air we observe is chiefly due to the weight, an intrinsic and inexorable property of all matter, of the gas. Yes, it occurs in sealed containers AND gas pressure is, fundamentally, always derived from the container walls. We can easily deduce that, IF there is an infinite sky vacuum above our heads (and even if there is not, just having constant air pressure at all necessarily involves containment), then there must be a barrier for us to have relatively static air pressure for aeons.
Deductions are often wrong, and I personally would like to see expeditions settle the matter.
@Tron1002
Have you heard of/seen skystone? It is bright blue due to its high oxygen content, and wouldn't you know it - ozone (and oxygen) is blue too for the same reason! They may possibly be related. Do you know the legends about operation fishbowl, and the first pixar short?