Need help in understanding Firmata applications

Hi guys. I've got some idea what Firmata is, eg. controlling an Arduino from a Processing language sketch on a host computer. (by loading the Firmata library into the Arduino)

My Question:

Can I have a running Arduino sketch downloaded into the Arduino AND modify variables in that running sketch via a Processing sketch running on a host computer?

For example, let's suppose I'd like to build and program my Arduino sketch to measure temperature and display temperature on an LCD... a fairly reasonable small project.

Let's suppose I don't have the physical temperature sensor (yet).

Can I have my Arduino sketch loaded in the microprocessor AND simulate the temperature sensor using Firmata from my host computer?

I just can't quite get if this is possible and if so, how to achieve it.

Any help will be greatly appreciated.

Didn't you already ask this here?