I remember some time ago Firefox denied all the local access as a security issue - despite all the dialogs that asked for rights. Had to find some other ways to access simple txt file - via link.
They didn't even want to give extensions that kind of capabilities (and in the process significantly crippled mass downloaders like DownThemAll and others), so yeah…
I was refering to ability to manipulate local files from Firefox, that existed 6 - maybe 8 years ago - long long time ago, when this filesystem API did not exist.