Can't save data on a local disc from Flash Player plugin in Firefox

Hello,

from some of the last Firefox version I can't save data from Flash Player applications.

Expected behavior that works in some of old Firefox version (I checked in v. 31):

1) I click on some button in a Flash application

2) a dialog window opens

3) I set a folder and a filename, click "Save"

4) success, I have a file on the local disc with a content provided by flash application

How it looks in the last version of Firefox:

1) I click on some button in the Flash application

2) a dialog window opens

3) I set a folder and a filename, click "Save"

4) some security popup appears that I don't have permissions to save in this folder and propose me to save in another folder like My Documents. Weird but I click OK

5) it transfers me to the My documents folder, I try to save in this proposed folder but those permission popup appears again and again until I click Cancel

6) as a result I can't save anywhere One addition - if I select some existed file (like I want to rewrite it) there is no permission popup, but flash player throws a Input/Output Error.

I googled this problem but haven't found anything similar, that's weird too..

So, I look forward for your help.

OS: Windows 7 x64

Browser: Firefox 49.0.1

P.S. in Chrome it works.