Dominique Hugo mentions on his site gosymbian.com that FExplorer is ready for Nokia S60 3rd Edition. He is facing a minor problem with the security message. One may need to turn off security certificate checking on the device.

Go to Tools?> App mgr?> Options?> Settings. Change ?software installation? from ?Signed Only? to ?All? and ?Onlline certificate check? to ?Off?.

And if I remember correctly, Symbian offers free “Symbian Signed” certificate for freeware applications