Test Firefox Portable Multiple Instance with URL/HTML Passing
I've put together an updated launcher for Firefox Portable that will allow passing of URLs and HTML files while in Multiple Instance mode. You'll need to set AllowMultipleInstances=true in the FirefoxPortable.ini file copied to the FirefoxPortable directory as usual. Download the ZIP and replace your current FirefoxPortable.exe with this modified version.
Download FirefoxPortableInstanceFix-2021-01-04.zip
Please test it out and let me know how it works.