I can confirm it doesn't work for me as well for both Linux (Arch and OpenSuse) on Firefox and Vivaldi.
But also doesn't work on Windows as well for same browsers:

Music is playing but still cannot recognize it:

Tried restarting the browser, reinstalling the extension, logging out and in again from Last.Fm and still nothing works.

Probably related to #4587. Works fine for me on Windows/Google Chrome + Chrome desktop PWA. The rework made the connector use the javascript api for playing media, is your browser correctly detecting the music?

Can confirm the same error on macOS + Firefox.

I see information in navigator.mediaSession.metadata while playing so it does seem to exist

though weirdly it's null if I set a breakpoint in the connector code, is that running in a different context?

on Windows 10 + Firefox same problem, Windows 11 + Firefox same, Manjaro Linux + Firefox the same

Have the same problem too on Windows 11 and Firefox

Oddly enough, this is working fine for me in Microsoft Edge only (Windows 10 and Windows 11) using the plugin version from the Edge Add-ons site. The other browsers I've tested (firefox, chrome, vivaldi) using the Mozilla or Chrome Web Store versions are reporting errors. The extension can submit now playing information but does not scrobble and reports a general error (!) from the extension.

Using the latest build artifact (Windows 11, Chrome/Vivaldi) listens/scrobbles correctly the first time. Now playing data is sent correctly and shown in the extension, but subsequent scrobbles/listens are "errored" in the cache. Selecting errors from cache and using the scrobble button does not work and deletes them from the cache.

Maybe related, but I get a "The track is not recognized" notification for every song, but it detects just fine on Google Chrome.

I did some testing and discovered, that the navigator Object that the extension accesses is not the same that the page accesses in Firefox.

I tried setting the navigator.mediaSession myself to debug, when setting it and then setting
track = navigator.mediaSession.metadata?.title, the extention show's what I have set, while the browser console still shows the song currently playing.
Even after removing the code that sets the mediaSession, it stayed as what I had set while the console on the page always showed the currently playing song.

It seems the extention gets it's own navigator Object from Firefox and cannot access the navigator the page uses, causeing the extention to be unable to detect anything on Firefox with the current implementation.

I suggest using selector classes like before #4587 since the version before still works or accessing the navigator object through an injected script