The installer available from the download page is an offline installer, in that you can download it and then run it later without an internet connection.
For the Zotero Connector, the Firefox extension is just an XPI that you can download and install later, and the Safari extension is bundled with the desktop app. Chrome extensions can only be installed from the Chrome Web Store.
Could you maybe just describe what you're specifically trying to do? You cannot download *anything* without the internet, so as posed, the question doesn't really make sense. Dan answered whether you can *install* Zotero without the internet, which you can -- including on a computer not connected to the internet by moving the downloaded installer, e.g. via USB stick.
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The installer available from the download page is an offline installer, in that you can download it and then run it later without an internet connection.
For the Zotero Connector, the Firefox extension is just an XPI that you can download and install later, and the Safari extension is bundled with the desktop app. Chrome extensions can only be installed from the Chrome Web Store.
Dan answered whether you can *install* Zotero without the internet, which you can -- including on a computer not connected to the internet by moving the downloaded installer, e.g. via USB stick.