Zotero is a Firefox extension, and installs just like one: visit the Zotero home page with Firefox, and click the red download button of the Zotero version you want to install. Then click the “Install Now” button in the Software Installation window that will pop up (the install button may be greyed out for a few seconds while Firefox downloads Zotero). Restart Firefox after the installation has completed. You should now see the “Zotero” logo in the status bar in the right bottom corner of your Firefox browser window.

By default Firefox automatically checks periodically for updates of installed extensions (which includes Zotero). Users can update extensions manually in the Add-ons window, which can be opened via the Tools menu, and clicking the “Find Updates” button on the Extensions tab. Note that users of Zotero 1.0.x will not automatically upgrade to Zotero 2.0. More information on upgrading from Zotero 1.0.x to Zotero 2.0 can be found on the Zotero 2.0 webpage.

During installation, you might run in one of the issues below:

• If it is the first time you try to install Zotero on your computer, you may receive the following message, appearing in a narrow yellow band at the top of the web page: “Firefox prevented this site (www.zotero.org) from asking you to install software on your computer.” If you receive this message, you'll also see an “Allow” button. Click this button, and then click “Install” in the window that pops up.

• After installation, the Zotero logo does not show up in Firefox. Solutions to this problem are discussed here.

• On rare occasions, you may get an error message. The most common of these messages is a “-228 error,” which prevents Zotero from downloading. There are a variety of reasons for this message, ranging from security software running on your computer to more technical considerations.

If you continue to have installations problems, you can try the following alternative ways to install Zotero:

1. Download Zotero from the Zotero home page by right-clicking the download button (on a PC) or control-clicking the “Download” button (on a Mac). Select the save option, and save the Zotero extension file (the name of this file ends in “.xpi”—be careful you don't save the image of the download button by accident) to your desktop. Finally, drag and drop that file onto an open Firefox window to start the installation.
2. A less preferred alternative (only possible for Zotero 1.0.x): try installing Zotero from the Firefox Add-ons site rather than from www.zotero.org. The disadvantage of installing Zotero from the Firefox Add-ons site is that you'll receive notifications of new versions of Zotero much later than users who install Zotero from www.zotero.org. (Every new release of Zotero has to be submitted to Mozilla for review, and it can take some time before updates appear on their site.)