Is this normal? I left my '91 C4 out in the 90 F sun and I came out to find my rear view mirror had fallen off the windshield and was sitting on the floor.

 

Nope. Not normal.

 

Not normal.
The rear view mirror is glued to the windscreen with some super-glue and is supposed to stick forever.
My car has been exposed to temperatures between -10 F and 105 F without the mirror falling off...

Have you replaced your windscreen recently?
If so, maybe they didn't use the right bonding material...

 

Not unheard of, but more than likely you have a replacement windshield, and the proper procedure for attaching the mirror was not followed. I have forgotten the brand, but I once used rear-view mirror adhesive that had a primer/cleaner and a tube of super glue. When I sold the car years later, it was still secure.

 

The mirror is attached to a mirror boss, round shape alloy thing. This comes attached to the glass and the mirror slides onto this boss.
You need to glue the boss back onto the screen, DO NOT use super glue. Super glue isnt flexible so expands and contracts at differnt rates to the glass and can make it crack. You need to use a 2 pack "Locktite" adhesive for glass 2 metal. Its expensive, so in the UK id go to Autoglass or sum1 similar.