If you know the app id of the package (like org.mozilla.firefox), it is easy. First to get the path of actual package file of the appId,
$ adb shell pm list packages -f com.google.android.apps.inbox
package:/data/app/com.google.android.apps.inbox-1/base.apk=com.google.android.apps.inbox
Now you can do some grep|sed magic to extract the path : /data/app/com.google.android.apps.inbox-1/base.apk
After that aapt tool comes in handy :
$ adb shell aapt dump badging /data/app/com.google.android.apps.inbox-1/base.apk
...
application-label:'Inbox'
application-label-hi:'Inbox'
application-label-ru:'Inbox'
...
Again some grep magic to get the Label.