i finally found a way to download a addon. just go to google and search for a addon
u will get link : https://getfirebug.com/releases/firebug/1.11/ go to that page and right click on .xpi file and click (save link as) it will now ask you to save the file. just save the file on your system and go to firefox->tools-->addons--> there near search u can see a dropdown and in that dropdown click-->install add on from file using browse.. thats all like this u can download addon and install anywhere.