I have observed there is a bug that prevents Sun Cult from working (at least under Mozilla Firefox 3.0.10) when the Firefox language is set to es-ES.
If the field "general.useragent.locale" in Firefox's about:config is set to a language other than es-ES, Sun Cult works.
It also works if we delete from the file chrome.manifest the following line:
locale suncult es-ES jar:chrome/suncult.jar!/locale/es-ES/suncult/
(deleting this line, the program simply can't find the es-ES localisation, so it proceeds to use the default English language.)
You can see that there are more people with the same problem, just check here:
I can provide more details on request.