Seems that extension v2.1.0 in Firefox 65.0.2 (64-bit) not work with YouTube Music. The debug log:
Bound to "neon.request" Shim.js:1:6422
Waiting for player to load... Shim.js:1:3534
Registered service: neon-extension:configuration <unavailable> Common.js:31:40300
Registered preference page: neon-extension:debugging <unavailable> Common.js:31:40300
Registered preference group: neon-extension:debugging:logging <unavailable> Common.js:31:40300
Registered preference option: neon-extension:debugging:logging:log_level <unavailable> Common.js:31:40300
Registered plugin: neon-extension <unavailable> Common.js:31:40300
Registered service: neon-extension-source-youtubemusic:configuration <unavailable> Common.js:31:40300
Registered preference page: neon-extension-source-youtubemusic <unavailable> Common.js:31:40300
Registered preference option: neon-extension-source-youtubemusic:enabled <unavailable> Common.js:31:40300
Registered plugin: neon-extension-source-youtubemusic <unavailable> Common.js:31:40300
Registered service: neon-extension-source-youtubemusic:activity <unavailable> Common.js:31:40300
Initializing "activity" service "neon-extension-source-youtubemusic:activity"... Common.js:31:40300
Initialized "activity" service "neon-extension-source-youtubemusic:activity" Common.js:31:40300
Bound to "neon.event" Common.js:31:40300
Player loaded Shim.js:1:3772
Received "configuration" event Main.js:1:4202
Received "player.state" event Main.js:1:4202
Configuration received Main.js:1:30436
[body] Start Common.js:1:6140
[null] Start Common.js:1:6140
[null] Add: <unavailable> Common.js:1:7499
[body] Add: <unavailable> Common.js:1:7499
[ytmusic-player-bar] Add: <unavailable> Common.js:1:7499
[ytmusic-player-bar] Start Common.js:1:6140
[ytmusic-player-bar] Duplicate node addition for: <unavailable> Common.js:1:7382
[.control-wrapper .middle-controls] Start Common.js:1:6140
[.control-wrapper] Start Common.js:1:6140
[.title] Start Common.js:1:6140
[.subtitle .byline a] Start Common.js:1:6140
[.subtitle .byline] Start Common.js:1:6140
[.subtitle] Start Common.js:1:6140
Received "player.state" event Main.js:1:4202
Player state changed to -1 Main.js:1:24648
Received "player.state" event Main.js:1:4202
Player state changed to 3 Main.js:1:24648
Received "player.state" event Main.js:1:4202
Player state changed to 1 Main.js:1:24648
Started progress emitter Main.js:1:27165
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 40.135999999999996 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 5035.532 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 10034.829 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 15027.392000000002 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 20061.224 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 25060.498 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 30051.95 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "player.state" event Main.js:1:4202
Player state changed to 3 Main.js:1:24648
Stopped progress emitter Main.js:1:27545
Received "player.state" event Main.js:1:4202
Player state changed to 1 Main.js:1:24648
Started progress emitter Main.js:1:27165
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 211281.383 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 216273.01499999998 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "player.state" event Main.js:1:4202
Player state changed to 3 Main.js:1:24648
Stopped progress emitter Main.js:1:27545
Received "player.state" event Main.js:1:4202
Player state changed to 1 Main.js:1:24648
Started progress emitter Main.js:1:27165
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 304241.36 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "player.state" event Main.js:1:4202
Player state changed to 3 Main.js:1:24648
Stopped progress emitter Main.js:1:27545
Received "player.state" event Main.js:1:4202
Player state changed to 1 Main.js:1:24648
Started progress emitter Main.js:1:27165
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 349241.156 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 354238.163 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 359238.276 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 364238.344 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 369240.498 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 374239.52300000004 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "player.state" event Main.js:1:4202
Player state changed to 0 Main.js:1:24648
Stopped progress emitter Main.js:1:27545
Received "player.state" event Main.js:1:4202
Player state changed to -1 Main.js:1:24648
Received "player.state" event Main.js:1:4202
Player state changed to 3 Main.js:1:24648
Received "player.state" event Main.js:1:4202
Player state changed to 1 Main.js:1:24648
Started progress emitter Main.js:1:27165
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 40.929 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Received "state" event Main.js:1:4202
Received player state: <unavailable> Main.js:1:27273
Media progress (time: 5035.827 ) Common.js:1:22978
No active session, ignoring progress action Common.js:1:23260
Seems that extension v2.1.0 in Firefox 65.0.2 (64-bit) not work with YouTube Music. The debug log: