I want to download yahoo messenger. But i am failure to do this. Becouse iphon need ios 6.0 to download yahoo messenger... That is my problem.. I cannot dowenload ios 6.0. Thank you..!!

This article should have everything you need.
How to downgrade iOS 8 to iOS 7 1 2

iPhone 3G cannot use iOS 6!
the latest iOS for iPhone 3G is: iOS 4.2
- iOS 6 is possible only on iPhone 3GS / 4 / 4S
iPhone 3G 8 16 GB Specs iPhone 3G A1241 MB046LL 2347 iPhone1 2 EveryiPhone...

The iPhone 3GS is partially supported by iOS 6. It is not supported by iOS 7 or subsequent versions of the operating system at all. iPhone 3GS 8 16 32 GB Specs iPhone 3GS A1303 MB715LL 2315 iPhone2 1...
Download iOS 6 1 6 for iPhone 3GS and iPod touch 4G

the new whatsapp app is not running on a iPhone 3G with iOS 4.2.1 as it is too old...
- but there is a workaround which you may try:
It is possible, a bit tricky, but possible. First, you'll need to download the Whatsapp app through iTunes on your computer, this will make it show up as "purchased" in your library.
After that, open up the App Store on your iPhone and go to the 'purchased' tab. When you try to install it this way it will tell you that you need iOS 6 or higher, but will also ask if you would like to download an older version of the app. then click on download.
- again step by step:
1) Open up iTunes on your computer
2) Go to Store (left hand column) ->iTunes Store
3) Search for Whatsapp; - download the app
4) then open App Store on your iPhone
5) Click on 'Purchased'
- you will have to install Whatsapp by clicking on it as usual.
App Store will ask you: "Download an older version of this app. The current version requires iOS 6.0 or later, but you can download the last compatible version.",
- then click on Download and it should finally work.

for installing the yahoo messenger into your ipad you have download the ipad version of yahoo messengerwhich must be compatible to ios i.e. iphone operating system.

You can download Yahoo Messenger for mobile from this link: http://mobile.yahoo.com/messenger
Simply select your type of phone (Blackberry, iPhone...) then put in your mobile number on the right side and click Send a link to my phone. Or type this URL into your phone's browser: m.yahoo.com/messenger

I CANNOT DOWNLOAD YAHOO MESSENGER BECOUSE FIREWEL IS NOT ACEPTED.WHY?