At this time it is only possible to use the extension on a chromium based browser.
Can be installed as follows:
settings -> more tools -> extensions -> toggle developer mode -> load unpacked
When you want to update the extension you should not uninstall the old version and then install the new version.
Instead you should replace the old files with the new files and click on the refresh icon for the extension on the chrome://extensions/ page.
To set your username/profile pic/github page, go to the [Settings] tab on the AOC website.
The extension will automatically take the username/profile picture that you have selected.
(If you have selected (anonymous user #XXXXX), no github page or profile picture will be sent to the server)
You can just browse AOC like you normally would, once you complete a challenge your progress is automatically saved.
Via the popup of the extension (click the icon in the menubar) you can join or leave a leaderboard.