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Why can't I install the Zotero Connector in Safari 12?
Apple has discontinued support for full-featured Safari extensions in Safari 12, which is the default browser in macOS 10.14 Mojave and also available for Sierra and High Sierra.
Safari 12 can only run Safari App Extensions1), a much more limited type of extension that must be distributed as part of a Mac app in the Mac App Store, which for technical reasons isn't currently a possibility for Zotero. Additionally, whereas the previous Zotero Connector for Safari shared a large amount of code with the Zotero Connectors for Chrome and Firefox, creating a Safari App Extension would require a near-complete rewrite of the extension, all to produce a much less featureful extension.
(Note that while Safari 12 describes all extensions using the previous framework as “unsafe” if you try to install them, this is simply a product decision on Apple's part. These legacy Safari extensions used an Apple-provided framework and worked essentially the same as extensions for Chrome and Firefox.)
For now, we recommend using the Zotero Connector for Firefox or Chrome instead. You can also use the Zotero Bookmarklet for basic saving from Safari. We hope to offer a better solution for Safari users in the future.
See this forum thread for further discussion on this issue.