Hmmm. Wonder if FF auto updated itself.
I did try this on my mac book pro with 10.13.6 and the installer also failed to install.
I'll check again with my version of FF on Mac.
I just checked with my Windows 10x64 PC and FF v52.9.0 ESR and I had previously installed the plug in as I was able to log into mydlink.com and access all three of my cameras on this PC.
Possible that the plug in isn't supported on later versions of OSX.

Please give the mobile apps a try...