Rated 3 out of 5 stars

Works fine with EN-US Seamonkey 2.33.1 (German Language pack) Gecko/20100101 Build 20150321194901 (Default Theme) on German WIN7 64bit. But to me the handling needs getting used to:
a) "Remove" Button always missing in add-ons-list
b) Checkbox "AMO integration" needs explication, I haven`t a clue what that might be for.
c) Checkbox "open in TAB" needs explication. Explication tells that "Add-ons Manager opens in its own window", but for me the default setting was "open in TAB",
what means that an additional TAB will be opened in Browser Window for add-ons-manager. Additional I think that without checkmark here the Add-on-manager
will not open in a new Window (what would be the same like a window with menu 'File -> New -> Browser Window'), but in a separate "dialog" (I am not sure whether that is an official name).

Reply to developer's comment April 14, 2015:
z) you are right, the missing "Remove" button is mentioned in add-on explication. But I do no t think that that is useful in a TAB ofa 1920x1080 screen

This review is for a previous version of the add-on ( 

That's actually deliberate. There's less space in a small dialog than in a full-screen window, it starts to look kinda busy with Disable/Enable and Remove for every single extension, and how often do you remove extensions anyway?

You can use the context menu, or double-click to open the full details page and use the Remove button there. Or if you really want Remove buttons in the main listing, use this in Stylish:

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
@-moz-document url(about:addons) {
.addon button.remove { display: inherit !important; }

Edit: Hey, did you know that AMO doesn't email me when reviews are edited? Yes, that's a fair opinion. I'm going to have to rewrite the whole extension to handle all the breakage in Fx40, so I'll see what I can do about these buttons while I'm at it. (It'll have to be an option of some kind, because I still think 3-4 buttons for every single extension is a bit too much.)