showSaveFilePicker with ServiceWorker randomly disconnects on Firefox

[Maxr1998]

I'm currently trying to get the showSaveFilePicker ponyfill to reliably work on Firefox. On Chrome, the native version already works with my code, and so does the library implementation when using _preferPolyfill: true. However, on Firefox (132.0.1 on Arch Linux), the download of a larger file (2.5 GB) randomly stops. It appears that the ServiceWorker is simply killed by the browser, which a regular ping should usually prevent, as far as I understand.

However, this doesn't seem to work, and looking at the code there might be a reason for that: in downloader.js, the keep alive uses setTimeout instead of setInterval, which seems unintended. Additionally, the postMessage call just passes 0 instead of a message like {type: PING}. If my assumptions are correct, I could look into creating a PR for this.