Disabling Schubert's PDF Browser plugin on Safari
I found a workaround. Move Schubert's plugin file which is located in:

/Library/Internet Plug-Ins/PDF Browser Plugin.plugin


or

/Users/YOUR_NAME/Library/Internet Plug-Ins/PDF Browser Plugin.plugin


to

/Users/YOUR_NAME/Library/Application Support/Firefox/Profiles/YOUR_PROFILE_DIRECTORY/plugins/


If you don't have plugins directory - create it. That's all :)

For Opera users - they should move plugin file to:

Library/Internet Plug-Ins/Disabled Plug-Ins/


Firefox and Safari don't use plugins from this location, but Opera does :)