My point isn’t about local file system access. OP said that if you’re making a complex app it should be native. I was pointing out that millions of people use complex webapps every day, adding file system access is a logical attempt to meet an audience where it is.

I'm not blaming Mozilla here (at least I don't think I should..? Not sure that the «enabler of bad behaviour» argument can really apply here ??)

I wish it wasn't a local filesystem. Google Docs was superior (for me) in that my files were available on every device. To achieve that here would require setting up Dropbox or similar and dealing with edge cases. If this API was more sophisticated with conflict resolution, Firefox could sync these files for me (maybe S3 compatible? I dunno) and then every app could have magical Dropbox/GDrive capabilities instead of a boring old tech folder that gets wiped out every so often.