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

Here is Why You Need An Adapter

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

French Guiana

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

Type C

Type D

Type E

Fiji

In Fiji, outlets of type I are in use.

Type I