You Do Need power plug adapter while traveling from Monaco to French Guiana.

Here is Why You Need An Adapter

The outlet and plug type used in French Guiana are C, D and E and in Monaco are C, E and F. This is how they look like.

Monaco

In Monaco, outlets of type C, E and F are in use.

Type C

Type E

Type F

French Guiana

In French Guiana, outlets of type C, D and E are in use.

Type C

Type D

Type E