Hey @Charumatibahen, firebug is no more in existence, it was stopped on March 2017. See details about it here.
Firepath extension has been removed for firefox. There is a better alternative, try chroPath.
ChroPath is light weight tool and a good user interface.
You can add chroPath extension in firefox from this link.
Here is the detailed screenshot of how to install and use ChroPath.
1. Click on the link given above.
2. Click on Add to Firefox. Click on Add.
3. Click on Ok.
4. Right click on the page you want to inspect. For me its www.google.com. Right Click --> Inspect Element[Q].
5. Click on ChroPath section----> Click on Inspect -----> Select the element you want to inspect-----> Select the locator you want and copy it.
Its done you have the locator, Xpath or Css Selector or Abs Xpath.
Hope this explanation helps.