Firefox, like most other browsers, has a horizontal tab bar. However, it might be better to have a vertical bar that can fit all the tabs on it more efficiently. You can add a vertical tab sidebar to Firefox with Tree Style Tab.
Tree Style Tab is an extension that adds a tree-style tab sidebar to Firefox similar to an Explorer folder tree. Open this page to add it to the browser. When you’ve restarted Firefox, you’ll find a vertical sidebar on the left as shown below. Note that it will also remove the horizontal tab bar.
Click the + button at the bottom of the sidebar to open a new tab. You can close any page by clicking the X button beside it. You can select tab context menu options much the same as the vertical sidebar.
However, the best thing about this extension is that it attaches pages open in new tabs to the one you opened them from. As such, it has a hierarchical tree format with arrows as shown in the shot below. Clicking an arrow beside a tab expands it to show you all the new pages open from it.
Open the Tree Style Tab Configuration window in the shot below by selecting Open menu > Add-ons > Extensions and then pressing the Options button beside Tree Style Tab. From there you can select a variety of alternative skins for the sidebar. Click the Top, Left, Right and Bottom radio buttons under Tab bar position to alter the position of the sidebar. Plus you can also select alternative tree twisties.
The tab sidebar remains open in the Firefox window by default. However, you can switch it to an auto hide instead so that it closes until you open it. Click the Auto Hide on configuration window, and then you can select the Auto Hide tab bar option there. That will remove the sidebar from the window, and you can reopen it by either moving the cursor to side of the window or holding the Ctrl key so longer as you’ve selected those options on the configuration window.
Overall, this is a great tab bar to add to Firefox. It will fit all your open tabs better than the default horizontal tab bar. You can also add vertical tab sidebars to Google Chrome.