Translation:I am bored to death.
Finally, a literal translation that completely works in both languages! :-) ^_^
But it's not exactly a literal translation. The literal meaning is it's so stuffy [in here] that I'm dying.
But what's the proper tone for 闷？ MDBG lists stuffy as first tone and bored as fourth tone.
That's right. It is a character with more than one pronunciation i.e. 多音多义字. (See RaphaelNing's answer for an example.)
Hmm, there's a small difference in the pronunciation of 闷 though: when referring to hot and humid weather, it's mēn; when referring to unpleasant mood (being unhappy or bored), it's mèn. BTW, "bored" can also be translated to 无聊, which has no other meaning.
Here another example of literal translation. In English you would also simply say "Iam so bored", "It's so boring" etc.... Take all these in...!!!!!