Mozilla
The majority of these pages sort of assume you know what Mozilla is. If not, then read on, otherwise you might want to skip to extensions (including the greasemonkey page), bookmarklets or sites for help and advice.
Basics
Mozilla make a variety of software, which is cross platform (Windows, Linux and Mac) and open source. They are best known for the Firefox web browser, which in my opinion is the best browser available today. They also make the email client Thunderbird, which has all the features and extensability you'd expect from Firefox's partner. Firefox and Thunderbird grew out of what was originally the Mozilla Suite (or just Mozilla). This was effectively abandoned in favour of the two separate applications but some prefer the integration offered by the suite, so it has been reborn as Seamonkey (this is part of the beauty of open source software). Mozilla also produce an ical compatable calendar application, either as an extension to one of the other applications or as the stand alone Sunbird. For more information (and free downloads) simply visit Mozilla or look at some of the sites listed below
Extensions
Part of the attraction of Firefox (and the other Mozilla applications) is the ability of the user to add the features they want using extensions. Many, many are available from both the official site and unofficial lists. It's also worth noting that there are also many themes to change the look of the applications. As you didn't ask, there's my personal list of must have extensions, separated into categories and with reasons!
Bookmarklets
Bookmarklets aren't a uniquely Mozilla thing, but they are useful. They're little pieces of javascript bookmarked so that opening that bookmark does useful things. I've collected some together which I find useful.
More Info and Help
The first place to rurn for help is the Mozillazine forums. Whilst not officially associated with Mozilla they are packed with knowledgable users happy to help and advice those who ask. Much information can also be found in knowledge base, including solutions to many of the most frequently encountered issues. There's also the Firefox help and Thunderbird help pages. For the latest Mozilla news try Mozillazine itself and lots of people blog about Mozilla.