I'd like to give this information here, because I searched for months for a solution and only now found the way for this user case:
Firefox Portable (v16.0.2)
PDF-XChange Viewer (v2.5.207) with Addin for Mozilla Firefox, Opera and Chrome
I updated both Firefox and PDF-XChange regularly, but while the Addin was functioning correctly for some time since some months (or a year?) opening PDF files in Firefox Portable showed them with a annoying red "DEMO Version,www.docu-track.com, Tracker Software" across every page.
There are several solutions for the normal Firefox indicating a version conflict of the Addin. Only now I realized (again) that I had to update manually the PDF-XChange Viewer Addin in the Firefox Portable:
copy the new npPDFXCviewNPPlugin.dll from c:\Program Files\Tracker Software\PDF Viewer\ to ...\Firefox Portable\Data\plugins\
Now everything is good *whew*
I hope Tracker Software keeps the Addin compatible for some minor updates and I don't have to repeat this for every PDF-XChange Viewer update.