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.
Marco Trevisan-Herraz