Help: spotlight can't see Firefox in Applications folder?
This has helped on a lot of macOS boxes I've worked on (10.14, 10.15 Catalina beta 1, beta 2, beta 3 and beta 4), perhaps for different reasons now in 2019 than when OP originally asked the question.


Open Terminal/iTerm and run:
sudo xattr -r -d com.apple.quarantine /Applications/Firefox.app
Authenticate with your admin password.


Your download was tagged by a macOS security mechanism as potentially dangerous, so the extended attribute com.apple.quarantine was set.  


Since the OS didn't ask for user confirmation, the xattr was most probably already removed, but only from the top level package Firefox.app itself, but not its contents. 
This is at least one scenario where an App  


can be launched without user confirmation, but 
Spotlight refuse to index. 



See superuser.com/what-should-i-do-about-com-apple-quarantine for more.