What does the code FU1FU2FU5 mean in this metar?
It's indeed the layer type and okta, but in the remark the layer type is repeated for each reported cloud (smoke) layer. From your example:

FEW037  →  FU1
SCT050  →  FU2  →  1+2=3
OVC250  →  FU5  →  3+5=8


The oktas of the layers are added, that's how OVC is shown as 5 and not 8, by adding the previous oktas it does become 8.

FEW = 1 to 2
SCT = 3 to 4
BKN = 5 to 7
OVC = 8


Similarly for CYCD:

METAR CYCD 012200Z 34010KT 12SM BKN038 BKN051 30/11 A3012
RMK FU5FU2 VIS LWR N-E SLP201 DENSITY ALT 1600FT=

BKN038  →  FU5
BKN051  →  FU2


When no remark is needed for the smoke (French: Fumée), the FU code is placed before the remark:

METAR CYBD 012200Z 29007KT 8SM FU FEW240 28/13 A3019
RMK CC1 SLP225 DENSITY ALT 1400FT=


According to Wikipedia, this is how Canada handles the remark section. Here is what navcanada.ca says:


  Where observed, the obscuration and cloud type amount for each
  reported layer is included in remarks. It may exceed 8 oktas if a higher
  layer is observed through transparencies in a lower layer


(Emphasis mine.)