If you're using Firefox, follow the instructions on this page to install Zotero.
If you're using Safari or Chrome as your browser, Zotero offers a version called Zotero Standalone that works with those browsers.
Choose Zotero for Firefox if:
Zotero for Firefox runs as a plugin for the Firefox web browser, not as a separate program.
The same version of Zotero for Firefox works on any computer that runs Firefox; there are no separate Windows and Mac versions.