The tabbed web browsers first appeared around 2004 when the first public version of the Firefox browser was also released. Since then nothing much has changed as far as the tabs are concerned in the Firefox browser – they are still being displayed near the top edge of the browser. But if you want to move them to the left or the right side of the browser window, then you can use the free Tab Center Redux extension for Firefox.
After installing the Tab Center Redux extension, you will see a new side bar with all the tabs listed in them. By default this side bar is displayed on the left side of the Firefox window. But you can click on the Tabs menu in the sidebar and choose Move sidebar to right to switch it to the right side. Similarly, you can select Move sidebar to the left to switch it back to the left side. Other than this, you can also hide the tabs being displayed in the side by closing the Tabs sidebar. If you want to display the tabs once again, then you can click on the Tabs icon in the Firefox toolbar.
The Tab Center Redux extension does not replace the tabs being displayed near the top edge of the Firefox window. It just adds another sidebar from where you can access the tabs in a much easier way. This works very well on the modern wide-screen or ultra-wide-screen computers.
In the settings for the Tab Center Redux extension, you can change the tabs sidebar into the compact mode which shows only the title of the webpages loaded in the tabs as opposed to displaying both the title and URL of the webpages in the normal mode. Similarly, you can switch back and forth between the dark mode and the light mode.
Conclusion: You can make full and complete use of all the space in your ultra-wide computer screen by switching the tabs to the side in the Firefox web browser.
You can download Tab Center Redux extension from https://addons.mozilla.org/en-US/firefox/addon/tab-center-redux/.