Mozilla uses a number of wikis, each with a different purpose. Over the past few years, a lot of work has been put into consolidating various sites and other resources into a smaller number. Here we look at the two most important.
Mozilla Developer Center (MDC)
MDC is meant to be a polished set of documentation vs. a scratch pad for random information. You can (and should) create your own account so that you can edit mistakes, or contribute new information. If you are unsure about a change you think needs to be made, you can ask in the #devmo irc channel.
Mozilla Wiki (wiki.mozilla.org, wikimo)
Where MDC is public facing documentation, the Mozilla Wiki (sometimes called wikimo) is a common collaboration point for organizing groups of developers, developing documentation which will eventually be moved elsewhere, etc. You can (and should) create your own account on the Mozilla Wiki so that you can participate in community documentation efforts, and also have a common collaboration point.
Wikimo uses the same software as Wikipedia (MediaWiki). That's where you can find the documentation on how to edit a page.