When opening .html / .url files or running FirefoxPortable.exe again I now get:
Close Firefox | Firefox is already running, but is not responding. The old Firefox process must be closed to open a new window.
My FirefoxPortable.ini reads:
[FirefoxPortable]
FirefoxDirectory=App\firefox64
ProfileDirectory=Data\profile
SettingsDirectory=Data\settings
PluginsDirectory=Data\plugins
FirefoxExecutable=firefox.exe
AdditionalParameters=
LocalHomepage=
DisableSplashScreen=true
AllowMultipleInstances=true
DisableIntelligentStart=false
SkipCompregFix=true
RunLocally=false