In the case of Selenium Locators to learn about Xpath and CssSelector its good to practice the Xpath and Css Selector on Firefox Addons like Firebug and Firepath . These Add-ons are compatible on Older Version of Firefox 55.0/56.0 and they are not compatible with Firefox Latest version Quantum.
To Download the older version of Mozilla Firefox we can access the link https://ftp.mozilla.org/pub/firefox/releases/.
To run the Selenium WebDriver Scripts we use the Selenium Jars , geckodriver.exe and Mozilla Firefox Latest Version which is Quantum.
So, can we have both Older version of Firefox and Latest Version of Firefox Quantum in the laptop.Is it possible???
Yes

Installing the First Version Mozilla Version – Latest
1. Access the mozilla firefox link . Click on Download button to download the executable file.

https://www.mozilla.org/en-US/firefox/new/

2. Open the executable file and complete all the Steps to download the Firefox in the C:\Program Files directory.
3. Double Click on the executable file to launch Firefox.

firefox.exe

Installing the Second Version Mozilla Version – Older Version 55.0/56.0
1. Download the older version of Mozilla Firefox 55.0 from this link.
https://ftp.mozilla.org/pub/firefox/releases/55.0/win64/en-US/
2. Double Click on Firefox Setup 55.0.exe to Launch the Installer.
3. Select the Setup Type as Custom instead of Standard.

Setup Type – Custom

4. Click on Browse button to select the Path to install the older version of Firefox.

Mozilla Firefox Browse button

5. Create a Directory under C:\ and select the Path.

Mozilla Firefox Installation Directory

6. Once it is installed double click on firefox.exe to launch the firefox browser.

Firefox 55.0

7. Navigate to Tools -> Options -> Preferences -> Advanced -> Update. Select the Option ‘Never Check for Updates’. Refer to the below screenshot.

Never Check For updates

8. Follow the link to Install Mozilla Firefox Add-ons[Firebug and Firepath].

http://total-qa.com/firebug-firepath-installation-selenium-webdriver/