It looks like funny things but in technical terms, you can prove a bit Genius in front of your friends. Changing User Agent in Firefox actually a trick to Modify default user agent of browser and Set Custom User Agent in FireFox.
One of mine friend visited a website using my PC and mine with a funny User Agent string. I used Google to find out how to change this string within Mozilla Firefox and found the answer on a Mac OS X forum.
The posting described a method of changing the string in Firebird, the old name of FireFox. The name Firebird was dropped by Mozilla, because there is also a database with this name.
How To Set Custom User Agent in Firefox-
It's not a big deal, you have to follow few steps as below:
1- Enter about:config in your Firefox URL area.
You will get an option just like above Image, click on accept and proceed it.
2- Right click on any highlighted area and select New > String as below image.
Now enter Enter the preference name "general.useragent.override", without the quotes.
Then Enter Your custom user agent value, for example, mine "TricksWay - Makes You Smarter"
Now Enter "about:config" in URL, you will see your custom User Agent as below image.
It's time to confuse web based sites and Google, just Google user agent testing sites and see the effect.