If you work with too many tabs open in Firefox, its not possible to see the URL of the next tab or particular tab on the tab bar unless he switches to or selects it, URL in Tab Tooltip extension for Firefox displays the URL of tab in a tab Tooltip when you hover mouse pointer on that tab (here you’re not selecting the tab). Similarly you can view other tabs URLs one by one also like this by hovering mouse pointer.

url of a inactive tab displayed in a tab tooltip

From the screenshot above, you can see I can able to view URL of ”URL in Tab Tooltip” extension tab by hovering mouse pointer over it rather than switching to it, here active tab is Gmail. You can download this extension from here.

I am sharing a bonus tip here, you can switch between tabs on tab bar by using keyboard shortcuts  Ctrl +1 for first tab and Ctrl +2 —Ctrl+9  for the last tab. You may know this but don’t forget to use these shortcuts when you’ve too many tabs open on tab bar and very difficult to navigate and select those first or last tabs.