Gecko needs to be installed to retest this ticket
In ReactOS 0.4.6 it was possible to install Flash Player 26 and 27 (current version at time of writing) from the usual URL. Firefox would download the installer and it would download the actual content - albeit usually only on second attempt. Then installation preceded successfully.
In ReactOS 0.4.7-RC1, configured the very same way, running the downloaded installer results in it failing to download the actual installation (screenshot attached). After the second attempt the installer will quit and delete itself (as usual). I have tested this multiple times, including after reboots.
It is interesting to note that while the screen (attached below) is shown, ROS runs on full CPU load, the "System" process is sometimes consuming as much as 113% of the CPU.