You Do Need power plug adapter while traveling from Fiji 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 Fiji is I. This is how they look like.

Fiji

In Fiji, outlets of type I are in use.

Type I

French Guiana

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

Type C

Type D

Type E