@Lakshmi, you can't find these plugins of Firebug and Firepath on Firefox Add-ons page because Firebug is no more in existence, it was stopped on March 2017. Also, Firepath extension has been removed for Firefox. Though there is a better alternative, that is ChroPath.
ChroPath is light weight tool and a good user interface. ChroPath generates unique relative xpath, absolute xpath, cssSelectors just in single click. ChroPath can also be used as Editor for selectors. It makes easy to write, edit, extract, and evaluate XPath queries on any webpage. You can add chroPath extension in firefox from here: https://addons.mozilla.org/en-US/firefox/addon/chropath-for-firefox/