I was having this same problem and was able to temporarily work around it by creating a Plugins directory under Firefox in AppData/Roaming/Mozilla and copying the NPSWF32.dll into it. It's located somewhere in the Windows directory. This also worked for me for other plugins having intermittent problems running in Sandbox mode in a 64bit Windows 7 install. Just find the appropriate dlls and copy them. But it doesn't work for Java. For Java I copied the entire Java install into the "Program Files" directory rather than "Program Files (x86)" and that seemed to help. But the Java plugin still seems to sometimes not load in the sandbox.
I'm **still** having issues with Flash sometimes *not* loading when using Firefox 4 (or 3.xx) in a sandbox.
I've tried the trick shown in the quote above and will see if it fixes it temporarily. But this issues has been going on for months if not more.
Is there any ETA on a permanent fix for this?
OR... has it been fixed but somehow my setup is bugged? Or do other people still suffer from this glitch?
Using Win7 x64 Home Premium.
P.S. doing a brand new install tomorrow or next monday (new PC) so I'll come back to confirm the problem still exists or not on new setup.