Funeral doom is basically a plodding,crushing wall of sound which,at times,can't even be classified as 'metal' save for the distortion.It oozes misery and despair and is definitely not for all times/moods.
Some bands would be:
Skeptcism,Thergothon,Winter,Tyranny,Enoch,Reclusiam,dISEMBOWELMENT(more death),Unholy,Funeral,The Funeral Orchestra,Church of Misery,While Heaven Slept,Until Death Overtakes Me,Dolorian,Shape of Despair,The Fall of the Grey Winged One,Mournful Congregation,Funerary Dirge,Eyes of Ligeia,Evoken,The Ethereal,Esoteric,Mausoleum(not the death metal band,this ones based in the UAE and have released only a three song demo so far),Hierophant,Hlidolf,Indesinence,Despond,Nortt,Wormphlegm,Quercus,Grief,etc.I'm sure I'm missing a ton too.Many bands have just demos out but a demo of funeral doom is a bit mroe than an EP of any other band.
A LOT of those bands are from out of Finland which has traditionally been the hub for extreme doom.However,it's really spreading.As mentioned,Mausoleum's from out of the Emirates and Church of Misery(a real killer)is from Japan.