You need to remove swfdec as it will clash with Adobe flash. You have to
pick one or the other, but since Adobe flash is more capable, that is
the one you'd logically choose.

Use the package manager to remove swfdec.

Back to Adobe flash, you say you "installed a plugin" but what exactly
did you do? What steps did you follow? I'm trying to ascertain whether
you just downloaded a flash tarball to your home directory, installed a
flash tarball system-wide, or installed a proper deb package.