Alternate download link?

Run the combo updater for the operating system you are using, and then Open Safari to download High Sierra.

Use the link found on this page:

https://discussions.apple.com/docs/DOC-12081

Make sure your data is backed up first.

The High Sierra link does work, do not use Firefox use Safari.

How to upgrade to macOS High Sierra - Apple Support

If the App Store says Open then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS High Sierra.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say 'get' or 'Download'.

Please note, if you are having issues with Firefox, and can reproduce its issues with the installer, it would helpful for all of us, if you could report the bug to:

https://developer.mozilla.org/en-US/docs/Mozilla/QA/Bug_writing_guidelines

For one thing Apple's Safari updates are limited typically to the last two operating system upgrades. And if you are applying updates after Safari's security has moved a step beyond you upgrade, you may not be able to use Safari to download the upgrade in question, and are left with alternate browsers like Chrome and Firefox.

Honestly I find managing password easier with it. And I got tired of running into sites that weren't compatible with Safari.

Yes, they should be, but just as Firefox can't load Apple, neither can Apple half the time load their own updates on their website with older version of Safari. Here's to the day I can afford a newer Mac of my own running Catalina. For now I just have my own work computer and my wife's.

Don't use Firefox for this link, it'll mess it up...

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

I gave up on Firefox years ago.

I gave up on Safari also.

Good news! :)