Can refer to either the Mozilla project or Mozilla Foundation. They produce a variety of software including Firefox and Thunderbird; thus, questions about a particular Mozilla program (versus the organization itself) should use a more specific tag.
A partial list of some of the more commonly-used pieces of software (and tags you should use for them) that Mozilla makes includes:
- Mozilla Firefox (firefox)
- Mozilla Thunderbird (thunderbird)
- Bugzilla (bugzilla)
- Seamonkey, formerly the Mozilla Application Suite (seamonkey)
More information about Mozilla can be read on Wikipedia.