If your Firefox Nightly blocks a download as a virus or malware, that's based on Google's "Safe Browsing", which can be disabled at Preferences → Security.


A block currently cannot be overruled, due to an incomplete UI: https://bugzilla.mozilla.org/show_bug.cgi?id=1048508


It might also block "uncommon" files (Maybe only on Windows? It is not completely clear what "uncommon" means.).


The most general bug on this is probably https://bugzilla.mozilla.org/show_bug.cgi?id=1053890 What I asked in https://bugzilla.mozilla.org/show_bug.cgi?id=1053890#c57 and below there is partly due to not having read some of the comments above it. Ignore the "FIXED" status.


Gervase Markham's comment (not fully up-to-date): http://blog.gerv.net/2014/09/google-safe-browsing-now-blocks-deceptive-software/