In recent versions of Firefox, the tabs are displayed at the top of the window by default. However, if you prefer to have the tabs at the bottom of the window, there are a few different ways to accomplish this.

Method 1: Using the Firefox Menu

1. Click on the three horizontal lines in the upper-right corner of the Firefox window to open the menu.
2. Select "Customize" from the menu options.
3. In the "Customize Firefox" tab, you will see a section labeled "Title Bar." Click the dropdown menu next to "Title Bar" and select "Tabs on Bottom."
4. Once you have selected "Tabs on Bottom," click the "Exit Customize" button in the lower-right corner of the window.

Method 2: Using the about:config Setting

1. Type "about:config" into the Firefox address bar and press Enter.
2. If you see a warning message, click the "Accept the Risk and Continue" button.
3. In the search bar at the top of the page, type "browser.tabs.onTop" and press Enter.
4. Double-click on the "browser.tabs.onTop" setting to toggle it from "true" to "false." This will move the tabs to the bottom of the window.
5. Close the about:config page.

Method 3: Using an Add-on

1. Click on the three horizontal lines in the upper-right corner of the Firefox window to open the menu.
2. Select "Add-ons" from the menu options.
3. In the "Add-ons Manager" tab, click the "Browse all Firefox Add-ons" button in the lower-right corner.
4. In the search bar, type "Tabs on Bottom" and press Enter.
5. Click on the "Tabs on Bottom" add-on and then click the "Add to Firefox" button to install it.
6. After installation, the tabs will automatically move to the bottom of the window.

In conclusion, changing the position of tabs on Firefox is a personal preference and can be easily accomplished through the Firefox menu, about:config settings, or installing an add-on. By following one of the above methods, you can move your tabs to the bottom of the Firefox window and enjoy a personalized browsing experience.