ff: Cleanest Addon Manager - original #userstyle got replaced with a #firefox addon (https://addons.mozilla.org/de/firefox/addon/cam/) but somehow I like the script-version more, so here is that

ff-cleanest-addon-manager.css

/* 
    original userscript got replaced with a firefox addon (https://addons.mozilla.org/de/firefox/addon/cam/)
    but somehow I like the script-version more, so here is a backup
*/

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

@-moz-document url("about:addons"),url("chrome://mozapps/content/extensions/extensions.xul") {

/*Compact top controls*/
#header{margin-bottom:7px!important;margin-top:-10px!important;}
#view-port-container{margin-bottom:-10px!important;}

/*Clean up add-on manager*/

#categories {min-width: 0 !important;}

 /*Category names*/
#category-discover, /*Get Add-ons category*/
 /*Stylish functions*/
.global-warning-container, /*Compatibility check warning*/
.addon > .warning, /*Incompatible warning*/
#list-view > #addon-list .description-container,
#list-view > #addon-list .relnotes-container
{display: none !important;}

.addon > :-moz-any(.warning, .error, .pending) > image {
margin: 0 -6px 0 0.2em !important;
list-style-image: none !important;
}
.addon .icon-container {
width: 16px !important;
height: 16px !important;
padding: 0 !important;
}
.addon .icon {
width: 16px !important;
height: 16px !important;
}
.addon :-moz-any(.name-container, .name) {font-size: inherit !important;}
.addon .content-inner-container {-moz-box-orient: horizontal !important;}
.addon-control {-moz-transition: none !important;}

#list-view > #addon-list .addon.addon-view {padding: 2px 12px 2px 7px !important;}
#list-view .addon.addon-view * {
-moz-box-orient: horizontal !important;
-moz-box-align: center !important;
vertical-align: middle !important;
margin-top: 0 !important;
margin-bottom: 0 !important;
margin-right: 0 !important;
white-space: nowrap !important;
}
}