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i'm not sure why they say mir ist heiss either, but i do know what "mir" is.

mir is the dative form of ich. it's kinda hard to explain, because i spent two years studying this in highschool, and i still have a bit of trouble with it, but there are 4 cases in the german language.

nomanitive
der-he
die-she
das-it
die-plural

accusative
den
die
das
die

dative
dem
der
dem
den

genetive
des
der
des
der


reflexive pronouns can be dative as well

I=ich-mich-mir
you=du-dich-dir
he,she,it=er,sie,es-sich-sich
we=wir-uns
you all=ihr-euch
they,you(formal)=sie,Sie-sich-sich

so when something is given or done for you or someone else, we use the dative form. the dative form is also used to show location, whereas the accusative form is used to show motion.

ex: i'll give you the shirt. Ich geb' dir das Hemd. dative
We're in school. Wir sind in der Schule (die Schule turns into der Schule because of the dative case)

but We're going to school takes the accusative case.
Wir fahren nach die Schule. because "die" in the nominative is the same as it is in the accusative, we left it alone.

but if we said The father is running after the thief, Father is in the accusative case, because he's the one that's running and the thief is in the dative case because he's being chased.
Den Vater laeuft nach dem Dieb.

another "mir" example is this:
i'm combing my hair. Ich kaemme mir meine Haare
! because the specific body part is labeled here (hair) the reflexive pronoun takes the dative case. remember, you're combing your hair for yourself is what it really boils down to.

i hope that helped DX

i didn't even know there was a difference. :O maybe it depends on what dialect you speak. i speak the northern dialect.