Win 7 laptop, Firefox Nightly, up-to-date, clean profile: Adobe CDM was not downloading at all -- I waited >15-20 minutes before giving up. In contrast, the Open H264 codec always downloads within about a minute. In the end, this was resolved by uninstalling+reinstalling Nightly, i.e. just updating my existing Nightly install was not enough.
Would love STR for this. Are you using a VPN or proxy? If you open about:addons > Plugins and right click on Adobe Primetime and select "Find updates", does the new plugin download. Also, you can set media.gmp.log.dump=true and media.gmp.log.level=0 and then you can view logging for the GMP downloader in the browser console (CTRL+SHIFT+J, filter on "GMP").
Ah, I just realized something: all is well on 32-bit Nightly; the CDM-download problem only arises in 64-bit Nightly.
No VPN, no proxy.
Ok, nothing to see here, move along.
(Because Adobe don't yet provide us with a 64bit GMP)