It's a special browser extension that plays specific tracks (MP3, OGG, etc) in the background when you visit a "location" in Fallen London.
By default, the extension is set up to play official FL OST by Maribeth Solomon and Brent Barkman. The idea is to replicate the original FL app experience, which it does with varying degrees of success. :)
NB: You have to provide your own copy of the OST (or any other music)! No tracks are supplied with the extension.
After the installation, you will see two buttons at the top of the game screen - "Locator" and "Mute".
The first one will display the panel shown on the screenshot with brief summary of where are you exactly and which music is set to be played at that location.
The second one will mute the current track and any other until you decide to "un-mute".
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