The About Add-ons extension adds an integrated button in the Add-ons window's list. It applies to Extensions, Themes, and Language packs, letting you quickly view the About dialog for the Add-on without using a context menu.
Since About Addon's final release for Firefox and Thunderbird in May of 2016, Mozilla has abandoned its own technologies in favor of poor parody of Google's protocols (WebExtensions), leaving many developers, including this one, disillusioned with the direction the Mozilla is taking. In contrast, Pale Moon has stepped into place as an XUL-centric browser, keeping backward compatibility better than Mozilla's own flagship project. So, for now, About Add-ons may be considered a Pale Moon and SeaMonkey extension that still supports older versions of Firefox and Thunderbird.