Q: firefox doesn't automatically open onsecondary monitor

Hello, baerb8!

Thanks for being a part of Apple Support Communities! It appears that your iMac on macOS Sierra and Mozilla Firefox aren't exactly cooperating together. The ability to open Firefox automatically on your second monitor isn't working. I'll be glad to help.

Try safe mode if your Mac doesn't finish starting up -- Safe mode will tell us if the issue is related to a startup item or plugins, and sometimes may be necessary to correct some problems. Test Firefox to see if it opens in the correct monitor. Reboot back to your normal user account and check the issue again.

How to test an issue in another user account on your Mac -- If the issue is also happening in safe mode, a test user will let us know if the issue is isolated to your account, or system wide. Let me know what you find.

Have a great day!

Thanks for trying those steps, baerb8. There's another step to use. Since Firefox didn't open in the 2nd monitor in the new user account, it may be that the issue is system-wide. To make ensure your iMac's operating system is working correctly, you can reinstall it.

Use Time Machine to back up or restore your Mac -- Before you begin, I always recommend a fresh backup to keep your personal information safe.

About macOS Recovery -- Follow this article to reinstall your iMac's operating system. Test Firefox again. 

Cheers!

macOS Sierra: Work in multiple spaces -- It may be a setting that needs to be adjusted.

Let us know how that worked and take care.