When an add-on is already installed, it should be indicated

[sarahmonster]

I was looking to install Firebug and ended up on a page that looked like this:

Greasemonkey is already installed & active, but this screen seems to indicate that it isn't. I'd expect that installed add-ons would look visually different—ie, maybe a greyed-out button that reads "already installed" or something similar. This would also reinforce that an add-on had been installed successfully—ie, when Firebug has successfully installed, the button changes state.

[wagnerand]

There are a couple of comments to this feature request already like https://bugzilla.mozilla.org/show_bug.cgi?id=1194183#c3

[andymckay-limited-access]

The new discovery pane now does this, we have the APIs. All we have to do now is figure out how to make it apply to all of AMO.

[EnTeQuAk]

I believe this will be reimplemented later anyway with properly showing the installation state so maybe this should move to addons-frontend or be closed and rescheduled for when we attempt to reimplement this.

[shell15]

we are expecting to do something like this in the next overhaul. cheers and thanks