Nb I did get a command line build running through Fire, without having to install anything else, by
export MSBuildExtensionsPath=/Applications/Fire.app/Contents/Resources/XBuildTargets
/Applications/Fire.app/Contents/Resources/Mono/bin/mono /Applications/Fire.app/Contents/Resources/Mono/lib/mono/4.5/xbuild.exe /path/to/your/sln /nologo /target:Build /verbosity:minimal /property:Configuration=Debug "/property:ElementsXbuildNougatSDKFolder=/Applications/Fire.app/Contents/Resources/Nougat SDKs" "/property:ElementsXbuildReferencePathsXMLFolder=/Applications/Fire.app/Contents/Resources/Reference Paths with Data Abstract Trial" /toolsversion:4.0