.SAFARIEXTZ File Extension
What is a SAFARIEXTZ file?
A SAFARIEXTZ file is an extension (aka plug-in) used by the macOS version of Apple Safari. It contains a feature that can be added to the Safari web browser, such as a spell-checking tool or additional search bar. Current versions of Safari do not support SAFARIEXTZ files.
Previously, developers could use Safari's Extension Builder (Develop → Show Extension Builder) to create Safari extensions. These extension development projects were saved in folders named .safariextension. After a developer finished creating their extension, they could build the extension into a SAFARIEXTZ file.
NOTE: Apple introduced Safari extensions in Safari 5.0; the ability to install and develop extensions was enabled by default in Safari 5.0.1 to Safari 12.
How to open a SAFARIEXTZ file
Previously, you could install the extension a SAFARIEXTZ file contained by placing it in the [user]/Library/Safari/Extensions directory. However, versions 12 and above of Safari do not support SAFARIEXTZ files. Instead, versions 12 and above of Safari allow users to install a curated selection of extensions by selecting Preferences... → Extensions → More Extensions....
If you want to view the constituent files a SAFARIEXT file contains, you can decompress the file with any Zip-decompression tool, such as Apple Archive Utility or 7-Zip. You may first have to change the file's extension to .zip.