Is firefoxportable.paf.exe corrupted?

My FF3 portable installation started acting flaky, so I downloaded it again from portableapps.com, and made sure to overwrite the previous .paf.exe file, but now I get a Windows pop-up "bubble message" (from the tray) saying it's corrupted, and now it doesn't appear listed in the PortableApps menu program, but when I try to run it manually (by clicking its icon) I get a "command window" for a second and then nothing.

Then I reinstall it again and it appears in the PA menu and I can run it, but when I remove the flashdrive and come back later for another session it's gone again and I have to reinstall again.

Really weird.

Thanks!