The issue starts from Firefox 69 and persists on Firefox 70.0.1. It used to work properly on old versions. However, an error window prompts from version 69 saying my profile may be missing or cannot be accessed. Is there a particular reason why Firefox Portable have to access %AppData%\Mozilla and %LocalAppData%\Mozilla in order to start up? If those permissions are not necessarily needed, could you please fix this? Thank you!
Firefox itself requires access to those two directories even when you tell it to use -profile from another location. You'd need to contact the publisher for that to be changed.
Sometimes, the impossible can become possible, if you're awesome!
Looks like official Firefox has changed the behavior since version 69. Thanks for the information!