NPAPI Plugin Samples
Collections of NPAPI plugin samples can be found in the Seamonkey source code at /modules/plugin/sdk/samples.
The samples may not build any more on all platforms. There are plans to clean up the sample plugin situation - better organization, updated build systems, get them building on all platforms. However, even if one cannot build the samples they can still be very valuable as code references.
In addition to those samples, there are 2 more plugins in the tree that might be helpful.