I've successfully installed Microsoft Silverlight 5 for Portable Firefox by installing Silverlight 5 as usual in C: and copying the contents within the folder C:\Program Files\Microsoft Silverlight\5.1.20513.0 and paste them in FirefoxPortable\Data\plugins. Everything works fine, and plugin status check is ok http://www.mozilla.org/en-US/plugincheck/.
What's puzzling is that even if I uninstall Silverlight from C: afterward, Silverlight still functions properly without Digital Rights Management Error whatsoever. However then, as I use Toolwiz Timefreeze, if I restart the PC, drive partition C will be returned to previous state (as if Silverlight had never been there), but the drive where Portable Firefox is located will be unaffected, Silverlight will still keep working except Digital Rights Management Error on Netflix.
The same happens with Chrome Portable
Please, thank you.