well you could install it manually. just take the *.xpt file and the *.class file from the tarball, copy them and paste them into your mozilla plugins folder.
my guess is you didn't tell the installer to install it into mozilla. the installer asks for a plugins path, you give it one, it installs, installer asks for another plugins path, etc... my guess is you installed it in another browser and then exited before giving it the path to the mozilla plugins folder. just a guess.