I actually think this is not an issue with FF in general but something I caused during dev somehow, because the exact same browser works fine on some of our other instances in production.
Edit: in dev it also works on other ports than 3000
Finally resolved - somehow I got a ton of data on my localhost and some unused ports from apps I developed a few months back still occupied localstorage space which led to the overall localstorage to overflow but it was just not clear from the error message.