It was working fine but in the last two Firefox Nightlies, it's stopped working completely.

[Jiaz]

I have informed macro. He will write here soon

[mrc]

Works fine for me (tested with 43.0a2 (2015-10-12)).
Can you create a ticket at support.jdownloader.org? Please describe the problem with a few more details.
What is the expected behavior? What was actually happening? How can I reproduce it?

I've done so. It sucks that forum credentials don't work on the support tracker but alas, I've filed anyway.

[Jiaz]

two different systems, that is why you have different logins