This isn't the list I set out looking for, but its pretty good...
All user preferences can be viewed and edited directly from the about:config page. This article is a partial documentation of those preferences.
Read more at Mozilla's KnowledgeBaseSome other useful links, especially if this doesnt make sense to you...
A Brief Guide To Mozilla PreferencesHow To Modify Hidden Preferences Using About:ConfigPreferential...
Welcome to Preferential! The Preferential project aims to provide a comprehensive resource for finding Mozilla preference documentation. It is also a popular Mozilla extension that provides all Gecko-based products (eg. Mozilla Suite, Firebird & Thunderbird) with a simple, accessible GUI to use when browsing and modifying preferences set within that product
I used to use a better system for checking what the preferences and there variables mean, it was in the MozillaZine knowledge base somewhere, but I cant find it n e more

Update: It all comes back to me know. If you put the preference name into the search box in the knowledge base, it throws back the page for that preference.