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.
Posted by: Vishal Gupta | Categories: Mozilla Firefox
Jaycee
I tried it on Firefox 4 beta 7 and it does not work!!,,,
Someone
This Firefox 4 Beta 8?
Mark Anthony
Wow.. COOL it works..
how to do this for permanent ?
KaaMoS
Nice easter egg's tricks!
Unfortunately the second one (Recent Updates) doesn't work when "Scriptish" is installed.
This add-on puts a tab under "Plugins", avoiding this zone to click.
shivaram
wow thats nice trick!!!
pyromania
haha.. nice one.. i wonder how he found out about this..
kashifmin
nice trick it works
Thomas
It does indeed work and is a good find, but those tabs are not really all that secret. The "Search" tab automatically opens when you perform a search at the top right of the Add-ons Manager, the "Available Updates" tab will appear when you untick the "Update Add-ons Automatically" menu item inside the cogwheel menu and the "Recent Updates" tab will open after you select "View Recent Updates" from that very same menu. It sounds more like a bug to me rather than an easter egg.
Ramesh Kumar
It works on FF4 Beta 7 (Official build)
Nice VG! thanks for sharing it.