I have no problem with FF 35.0.1 and I see the same version number of the CDF plugin.
Is the plugin enabled in FF Add-ons Manager-Plugins?
Supposing you are on Windows, have you tried quitting Mathematica through the small icon in the notification area?
I think that the plugin doesn't work if there's some Mathematica kernel active (or launched by the Mathematica program or, maybe, left running from some other browser).