GX Mods by Noble, Taylor Robinson

Do you like sound effects? I sure do.
GX Mods (GXM) allows you to have Opera GX style sound effects and background music, from within the comfort of Firefox.
It comes with some example mods, but you can make your own or download more from the actual GX Store.

Disclaimer: I am not affiliated with Opera Norway, Opera Software, or Otello Corporation in any way, shape, or form. Please don't expect any support from them.
If you need support, head to the support website of the extension instead!

IMPORTANT NOTE: The addon currently requires to be run in a normal window for full functionality. In Private Windows, Firefox doesn't allow the use of IndexedDB, which breaks mod installation and loading.
If you use Private Windows frequently, you'll need to open a normal window to install mods and change your settings.
Additionally, if Firefox is set to use Private Browsing on startup, any mods that aren't built-in will cease to function.
Sorry for the inconvenience, there may be a way around this at some point in the future.

Finally, you might notice GXM requires quite a few permissions! Here's the breakdown for all of them.

🌐 Access your data for all websites: Required so GXM can detect page events like mouse clicks and hovers to ramp up the music.
GXM won't read your cookies or anything like that.

📥 Download files and read and modify the browser's download history: Required so GXM can ramp up the music when downloads finish, if you so choose.
GXM won't download stuff or rewrite history. It only checks when downloads finish.

🔔 Display notifications to you: Required to tell you when mods are installed, and what content has been added.
GXM won't notify you for anything other than that.

📂 Access browser tabs: Required to ramp up the music and play sound effects when tabs are opened and closed, plus disable the music in certain situations.
GXM will only care about the URL you're on if you have "Disable music while shopping" enabled. Even then, it's just used to mute the music.

⏳ Access browser activity during navigation: Required to ramp up the music when pages load, if you so choose.
I don't know what kind of shady thing you could do with this permission, but whatever it is, GXM won't do it.

💾 Store unlimited amount of client-side data: Required to store downloaded/created mods.
Without this permission, the extension would stop working after you download too many mods.