When trying to open the web interface of my Thecus N4200 nas I get the message: We're sorry, Lightspark encountered a yet unsupported flash file

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Linux localhost.localdomain 3.14.7-200.fc20.x86_64 #1 SMP Wed Jun 11 22:38:05 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

Description of problem:
When trying to open the web interface of my Thecus N4200 nas I get the message:
  We're sorry, Lightspark encountered a yet unsupported flash file
  SWF file: http://nnn.nnn.nnn.nnn/login.swf
  Cause: Unhandled ActionScript exception
  Press Ctrl+C to copy this error to the clipboard

(that Ctrl+C does not seem to be functional in Firefox btw)

Version-Release number of selected component (if applicable):
INFO: Lightspark version 0.7.2 Copyright 2009-2013 Alessandro Pignotti and others

How reproducible:
100%

Steps to Reproduce:
1. Open web interface (login.swf) of Thecus N4200NAS

Actual results:
Message show above... We're sorry, Lightspark encountered a yet unsupported flash file

Expected results:
Login screen

Additional info:
- Konquerer shows login screen but does mot allow login (greyed out).
- Google Crome shows http://nnn.nnn.nnn.nnn/index.php login and allows for login without apparent problems...

... I prefer to use Firefox and not Chrome though.