It is very easy to open more tabs than you close until you have lost all sense of how many you have open. This simple add-on displays how many tabs are open across all Firefox windows. The total is shown in a "badge" on a button that appears in the toolbar by default. (The button does not do anything when you click it.)
Click the button to see a panel that indicates how many tabs have content loaded in them. (If you quit and then re-open Firefox and have it set to show your tabs from last time, then each tab's content is only loaded when you first click on the tab and make it active. So if you have a lot of tabs open, most of them may not have any content loaded.)
Note that after installing this add-on the number of loaded tabs will not be accurate until you restart Firefox. The number of open tabs, however, is accurate immediately, without a restart.
(Also note that for obscure technical reasons some of Firefox's "about:" pages such as "about:newtab" are not always counted as loaded content.)
