Can't find uninstaller in Folx downloader.mac.dmg
OS X is a bit different. After putting the application itself in the trash, you should probably use safari preferences to remove the extension. Once you do that you do that, you might want to log out and back in.  Then empty the trash. 

Alternatively you can look in the library folders - use finder go menu and choose "Go to Folder" and look in both below locations:

~/Library/Internet\ Plug-Ins
/Library/Internet\ Plug-Ins


Note the space is encoded with a \ but you can skip that most of the time if you are not pasting into terminal or scripting. Delete any plugins to be nuked and reboot if you removed any from the second system library folder. Then empty trash.