there seems to be a bug regarding "PortableFirefox.ini".
Setting the variable "UserProfileDirectory" to a directory has no effect. On the other hand, if you define an "AppDataDirectory" everything works fine.
Unfortunately, the variable "AppDataDirectory" is not mentioned in the file "Readme.txt" (any more).
You're probably using an old version of the launcher with a copy of Firefox that auto-updated. Check the help.html file in the FirefoxPortable directory. If it says less than 184.108.40.206, then you are using the old buggy launcher that had this issue. This is a known issue that has already been fixed. To upgrade your launcher, you can upgrade the whole thing or you can download the launcher from SourceForge and manually copy your old FF and profile into it.
Sometimes, the impossible can become possible, if you're awesome!