The issue is that SeaMonkey never had as big of a following as Firefox, and thus didn’t have as many extension developers. On top of that most FF extensions never included the required navigator overlay needed to display/work properly on SeaMonkey. Basically the user interface is much different than that of FF's and needs extensions written for it. Since spiderweb/sealion/snowlion are all using the seamonkey UI most FF extensions wont work properly, if at all without some tweaks. That’s why i tend to include or link to popular extensions that have been modified to work.
Cheers
Thank you for that explanation.
The missing detail I lacked previously is SnowLion is a fork of SeaMonkey, not a fork of Firefox.
Although I sometimes forget about the way Mozilla dropped the Communicator-descended applications suite in favour of stand-alone clients (FF, Thunderbird, etc.), your reply clarifies a great deal.