So this is only a preliminary patch because it has a few problems that still need addressing:

1. MediaSession works like a charm. As noted in the comment, we might need a strategy to get the best artwork (highest resolution? closest to square dimensions? etc
2. The default platform artwork would need to find the current installation directory (here I've taken my ubuntu path but still with a typo () ....))
3. Since currently the higher API is passing me the svg file to , but that path doesn't exist for me, it stops at mozapps, I don't have places.
4. The above path (icons/default) is not available in my build, only in real distributions, so we need to check those conditions, this is specifically relevant for other brandings.

First, now I still don't know what the best way is to requery the file location of branding icon. If the user modifies their diretory name, such as renaming to then this code would pass an invalid path to user. Second, now we have guaranteed that artwork won't be empty, if we want to replace the favicon icon with branding icon, we should modify the code in [1]. Also, if we would like to use the branding image, we should check what current version Firefox we're using and then select the correct image.

[1] https://searchfox.org/mozilla-central/source/dom/media/mediasession/MediaSessionController.cpp#177-200

@baku we would like to show the branding icon on the virtual control interface that system provides. Do you know what the best way is to get the file-based URL for those branding images? Or you know who we can ask about this topic?
Thank you.

BTW, will we have a part2 for this bug? I guess you would implement the SMTC part in part2?

Exactly, I am just working on the Windows Part to specify a local file.
Part 3 could then be changing the branding icon etc, so this is a WIP.

Edit: " Second, now we have guaranteed that artwork won't be empty, if we want to replace the favicon icon with branding icon, we should modify the code in [1]"

Note that the MediaController in 1 probably doesn't know about the individual packaging of the user agent. For me there is a big difference between Ubuntu and Windows. But that all depends on what we can come up with, maybe there is a better solution from baku and others.

According to commet7 and comment8 [1] in bug1623971, we should not let external media framework load an URL because of both security and private concern. I will file a bug later to block this bug, which would provide a way to get an image bitmap from an image URL. That could be used for SMTC, but for MPRIS, we should only set the filed-based image, because MRPIS doesn't support an image bitmap as an input.

[1]
https://bugzilla.mozilla.org/show_bug.cgi?id=1623971#c7
https://bugzilla.mozilla.org/show_bug.cgi?id=1623971#c8

We've found another solution and the bug is closed, so let's abandon this revision