file linked from https://portableapps.com/apps/internet/firefox_portable/test gives installer file not found error constantly with several tries. I guess eventually things will sort out, but for now nightly would probably be better off linked to previous version or be fixed preferably.
This will happen whenever there is a version switch. The links that Mozilla uses all change, so the live installer can't find it. It usually happens within 48 hours of each new stable release.
The new beta, aurora and nightlies are building now and will be available soon...
UPDATE: And they're all updated.
Sometimes, the impossible can become possible, if you're awesome!