Debian Stable vs Testing vs Unstable
 

I am confused about the different types of Debian OS offered by Debian community. What do you mean by them ?

What testing do they perform ? Are they on the kernel code or the software installed on the OS or just the compatibility testing with various hardwares ?

If I install Debian Stable, then Couldn't I get latest versions of the softwares like firefox using this command "apt-get install firefox" ?

What are the advantages of all the 3 Debian OSs?

https://www.debian.org/doc/manuals/d...g.en.html#s3.1
https://wiki.debian.org/DebianTesting

To expand a little, Debian only has ONE release - stable (currently called Jessie).
The 'testing' repo is the construction zone for the next stable, and may or may not be in a really usable state at any given time (this generally calms down a bit as the next release approaches).
The 'unstable' repo contains the pool of "current" packages which, as it's name implies, change frequently - some of which will migrate into testing as the various criteria for inclusion are fulfilled.

If the debian repository has the package for Firefox, you can definitely install it using apt-get install command. You have to check the debian repository whether the firefox deb package is available or not.

As Debian is that cool we freezed the firefox to an iceweasel: https://wiki.debian.org/Iceweasel

Also, Debian uses the ESR (Extended Service Release) version of Firefox for their current distro's. So currently if you do you will get v38.3.0, not 41.0.2 which is the latest. In order to get the absolute latest firefox, you'd need to install the binary version of actual Firefox (which isn't difficult), or run testing/sid and use the experimental repository which is not suggested for beginners, or anyone that doesn't know how to repair their system when updates break multiple programs, or sometimes even the entire desktop.