It is very simple and easy process to install fire bug on any version of Mozilla Firefox
1. Please visit this fire bug page https://addons.mozilla.org/en-US/firefox/addon/firebug/
it will look like this image
2. Click on add to Firefox button and save link as … and save the .xpi file.
3. Now open Firefox and click on file menu and then open file and browse that .xpi file and then install that add-on .
Note :- Also you can install any firefox addon by following these steps easily without an internet connection.