There's a bug in Firefox (if you want to call it that) where they won't support "Shared Buffer Arrays." I think it's actually a decision on their part because Shared Buffer Arrays can be a bit sketchy in terms of security. Chrome, however, supports them. In Itch, the problem is that you can't tell it to selectively pick whether or not to support Shared Buffer Arrays: you have a project that either supports it or not. I could have just uploaded a version without support for Shared Buffer Arrays that works in both Chrome and Firefox, but I prefer the higher fidelity audio that you get in Chrome (which is what I use) due to the Shared Buffer Arrays, so I tend to upload two different projects.
The reason this happens is Godot itself has the ability to do multithreading and it's an option you check when exporting, but if you check it, you have to have the Shared Buffer Array support, or it won't run in your page. So I have two web exports: one for with Threading and one without, and then upload to two different pages. Multithreading in web exports is used to make the audio fidelity much higher, with less crackling.
I'm genuinely glad to know people are taking advantage of that, though! Maybe in the future Itch will give us the option to autodetect via the browser choice, and have two different uploads depending on which is found.