Completely not works for me, even after install the component in "windows.zip".
Check connection always told me no native client . Windows 7 x64 + FF57

Somehow, I found out that FDM 5.1.33 has been released, as it claims working for FF57.
After FDM 5.1.33 installed & enable extension in its setting again, everything works as old day.
https://image.prntscr.com/image/VM7RUK25TN6VTN10CmCImQ.png

FDM 5.1.33 official release
https://www.freedownloadmanager.org/download.htm
http://www.freedownloadmanager.org/board/viewtopic.php?f=1&t=17363&p=41704
- Added Firefox 57+ support.