Zotero Connector and Safari 12
You can install the Zotero Connector for Safari 12 from the Safari Extensions Gallery:
Due to changes by Apple, we will not be able to update the Safari extension after December 2018, though the version in the Extensions Gallery will likely continue to work until Safari 13 is released in fall 2019. We're evaluating our options for best supporting Safari users going forward.
Apple has begun to discontinue support for the original type of Safari extension in Safari 12, which affects the Zotero Connector and many other extensions.
These Safari extensions, which work similarly to extensions for Chrome and Firefox, are being phased out in favor of Safari App Extensions, a much more limited type of extension that must be distributed as part of a Mac app. A Zotero Connector written as a Safari App Extension would offer much less functionality than the existing Safari extension, which is already less powerful than the versions for Chrome and Firefox due to the more limited framework.
In Safari 12, it's no longer possible to install Safari extensions distributed directly by developers.1) It's still possible to install legacy extensions from the Safari Extensions Gallery, but submissions will only be accepted there through December 2018. While we've avoided the Extensions Gallery in the past due to long approval times, we've chosen to submit the extension in order to continue providing the Zotero Connector to Safari users for the time being.
We don't know how long Safari will continue to support extensions from the Extensions Gallery after December 2018, and the inability to push out updates will prevent us from being able to offer new features or fix bugs.
For the best Zotero experience, you may wish to use the Zotero Connector for Chrome or Firefox instead. Firefox and Chrome provide powerful extension frameworks that allow us to offer advanced functionality such as automatic proxy detection and automatic RIS/BibTeX import, features that weren't possible in Safari even before Apple further restricted what Safari extensions can do. We're also able to release updates to the Chrome and Firefox extensions immediately, rather than waiting for updates to be approved by Apple.
Regardless of future Safari extension support, you will always have the option of saving to your Zotero library using the Zotero Bookmarklet, but we're currently evaluating our options for best supporting Safari users going forward.
See this forum thread for further discussion of this issue.