Refer: https://portableapps.com/node/23783
WORKS!
This solution was given by other authors as well - but i want to repeat and emphasis that it REALLY WORKS - and its simple as 1-2-3:
- From the directory where your "FirefoxPortable.exe" is (let's call it appRootDir) you go to directory "Other\Source\"
- copy the file "FirefoxPortable.ini" to the appRootDir
- change line "AllowMultipleInstances=false" to "AllowMultipleInstances=true" in the copied "FirefoxPortable.ini" in appRootDir
Sample Location:
C:\Users\mruckman\Documents\PortableApps\PortableApps\FirefoxPortable\FirefoxPortable.ini