Now that's pretty interesting thing. One AskVG reader "JrColas22" found 3 secret hidden tabs in Add-ons Manager of Mozilla Firefox 4.0 Beta:
To access these hidden tabs, follow the instructions given below:
1. First open Add-ons Manager in Mozilla Firefox 4.0 Beta. You can open it by clicking on the orange "Firefox" button given in Titlebar and selecting "Add-ons" menu or by pressing "ALT" key in keyboard to show Menubar and selecting "Tools -> Add-ons" menu.
2. Now to access the hidden tabs, do following:
a. To access "Search" tab, click a few pixels above "Get Add-ons" tab.
b. To access "Recent Updates" tab, click a few pixels below "Plugins" tab.
c. To access "Available Updates" tab, click a few pixels above "Recent Updates" tab.
3. Don't know why these tabs are hidden by Mozilla team but its really interesting to reveal them. ;)
PS: This method has been tested on the latest Mozilla Firefox 4.0 Beta 8 Preview build. Don't know whether it works on previous builds or not. Please follow and confirm it.
UPDATE: "Search" hidden tab actually appears automatically when you search for something using "Search all add-ons" searchbox in Add-ons Manager. "Recent Updates" hidden tab can also be revealed by selecting "View Recent Updates" option from menu near searchbox. It seems "Available Updates" tab would appear automatically when an update is available for any installed add-on.