Mozilla.org doesn't exactly hide information on the spell check project, however they don't give you a clear road map to the download nor do they provide good install procedures.

Part of the reason is that the spell check remains under development. The download site warns of possible bugs. I haven't had time for a thorough test, however early use has been good.

Current plans are to include the spell check in the mozilla 1.3b release.

Anyone using Mozilla and wanting a spell check "now" should do the following.

Download the appropriate spell check module for the version of Mozilla that you are using. I am running Version 1.3a.
Download

The download will be a .xpi file. To install, open that file with Mozilla by clicking "file>open file" or "Ctrl+O".