Max Tabs is an add-on for Firefox that allows users to set the maximum number of tabs that could be opened in any Firefox window. If you are asking “Why?”, the answer is simple: to bring order into tab madness. If you have the habit of forever opening new tabs without closing the ones already open (something that happens to me often when reading Wikipedia articles) you will agree that it’s a good idea to have someone monitoring over you.
Another reason why you may want to limit the number of tabs is if memory is scarce. By limiting the number of pages you open, you limit the amount of memory Firefox consumes which makes the browser less prone to crashes and improves responsiveness.
By default Max Tabs the maximum number of tabs opened in a window is set to 10. The extension keeps count of the number of opened tabs on the lower-right on the addon bar. If you try to go over the limit, Max Tabs displays a message “You’ve decided not to open more than x number of tabs” and promptly closes the new tab.
If you decide you really need to open the extra tab, you have to disable Max Tabs. Thankfully, this is easy as Max Tabs is a restartless add-on. You can swiftly enable or disable the add-on without restarting Firefox.
Also read how to Load Tabs Progressively in Firefox