This issue only occurred on one of my two computers where Firefox Portable is installed, so I'm assuming it was a fluke file corruption or the like. But here is the full popup message:
Integrity Failure Warning: Mozilla Firefox, Portable Edition was installed or upgraded without using its installer and some critical files may have been modified. This could cause data loss, personal data left behind on a shared PC, functionality issues, and/or may be a violation of the applications license. Neither the application publisher nor PortableApps.com will be responsible for any issues you encounter. Would you like to start Mozilla Firefox, Portable Edition in its current unsupported state?
I clicked No. A second popup followed:
Would you like to visit the Mozilla Firefox, Portable Edition homepage to download the app and upgrade your current install?
I clicked No again, backed up my profile sub-directory, and was going to re-launch or re-install, but decided to just report it to the forum here for feedback instead.