Mozilla provides the http://addons.mozilla.org (AMO) web site to provide a repository for add-ons for Mozilla software. When users click the "Get Extensions" link in the Add-ons window in Firefox, for example, they are directed to this site.
That makes AMO a great way to get your add-ons to the public. This article provides details on how to submit your article to AMO for distribution.
This is important. It's hard to get an add-on accepted into AMO if you don't write it first. Really hard.
Make sure it works before you submit it. You should try it out on every product you claim to support. In other words, you don't want to advertise that it works in both Firefox and Thunderbird if you haven't tested it in both. Make sure it works in every version you claim to support, too.
Add-ons distributed by AMO need to be packaged properly as XPI files. See Extension Packaging for information on how to do this.
You'll need to have an AMO account so that you can make submissions. To get one, visit the Register link at the top of any page on the AMO website. Fill out the form and follow the instructions to activate your account.
Obviously, you can skip this step if you already have an AMO account.
To submit an add-on, go to the Developer Control Panel.
You will then be asked to supply a file, as well as information about the add-on.
Once the add-on has been reviewed, it will be made available for downloading. Reviews can take a varying amount of time depending on how many pending submissions there are and the availability of people to perform the reviews.