As stated above, this is a bogus bug.

[not a bug! for those who don't understand the word "bogus"]
The browser SHOULD display an error, since when you ask to save the file, the server doesn't reply with OK, but with DENIED (that is 403).
I don't know why Firefox ignores this fact. I think it's a usability problem, if not a true bug.
In my opinion someone should report this problem to firefox team, as a bug report.
In the past someone else already reported this, and i was worried as i was today.

I always forget about it.
Thank you anyway for reporting.