Firebug addon in firefox for developers -- used to modify css, html and java script

Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page....

Default Installation Procedure

To install in goto Tools -> Add ons 

then in the search box type firebug and from the obtained search list select firebug and choose install.

after installation the firefox need to restart to apply firebug.

Manual Installation :

1. open any indexes of version u want,http://getfirebug.com/releases/firebug/1.6/
2. right click on any of the link and choose "save as"
3. There will be one ".xpi" file which l b downloaded locally to ur computer.
4. Copy tat .xpi file and paste it in ur computer/server where there is no internet connection.
5. In that PC/sever,open firefox-->go to tools -->add ons
6. Drag tat copied .xpi file into add ons window and click on install
7. restart firefox ur work is done :)