Extensions won't enable in Safari 11.1

Try the Safe Mode to see if it works there. You can also try going to Safari/Preferences/Advanced to remove the Develop menu from the menu bar, then quit and reopen Safari. Enable the Develop menu and if necessary, see if you can remove the check mark.

Safe Mode - About El Capitan
also Sierra

Safe Mode Startup – El Capitan. also Sierra

Might be a corrupt .plist.

Do a backup, preferably 2 backups on 2 separate drives.

Quit the application.

Go to Finder and select your user/home folder. With that Finder window as the front window, either select Finder/View/Show View options or go command - J. When the View options opens, check ’Show Library Folder’. That should make your user library folder visible in your user/home folder. Select Library. Then go to Preferences/com.apple.Safari.plist. Move the .plist to your desktop.

Restart the computer, open the application, and test. If it works okay, delete the plist from the desktop.

If the application is the same, return the .plist to where you got it from, overwriting the newer one.

This may require a reset of some other Preferences.

Thanks to leonie for some information contained in this.

I have the exact same problem - occurred after installing the 10.13.4 Mac OS updater today. Have been scouring the Internet looking for a resolution. Can't find one, have tried everything from Terminal commands to editing the Safari plist with Xcode. My final workaround now is to set Chrome as my default browser. I can't live without extensions, especially password manager Dashlane. Cheers, B

Weird - I rebooted and the issue has magically disappeared. Perhaps my tweaking of the safari.plist file with Xcode somehow worked. Or a moonbeam hit my Mac. In any event, happy to have my extensions back in working order.

Had the exact same problem. Ended up doing a reinstall as well. I have no idea what could've caused it but I'm guessing the OS update could've been the culprit. I did the update and the system worked fine until a day later when I restarted my Mac and opened Safari to find the extensions gone.

Just experienced this problem today. It seemed to happen (or at least I first noticed the problem) shortly after uninstalling the Adobe Flash player from another admin account on the computer.

It was impossible to remove the check next to the "Disable Extensions" option on the Developer menu from my account

After two reboots the extensions were enabled and working again. Very strange.

Try going to Safari/Preferences/Extensions to see if you can enable them there.

Try setting up another admin user account to see if the same problem continues.

I, too, am spontaneously experiencing this after updating to 10.13.4.

I can confirm, though, that other users of my machine *do not* experience this issue.

Had the same problem.

Preferences - Advanced - Internet plug-ins - Uncheck "Stop plug-ins to save power.

Worked for me.

:-)