SuperSafe Boost from f-Secure is supported on Safari 8 to my knowledge.
The issue with Safari 12 is down to Apple. Apple have said that Safari 12 deliberately blocks support for legacy Safari extensions that come from outside the Safari Extensions Gallery. Apple are restricting what you can do with the Safari browser to Apple approved extensions. I've yet to see any comment from f-Secure that they're going to be able to work around this Apple restriction.
Fortunately, other browsers are available.
Just installed Safari version 12.0 myself and having exactly the same problem. I have no issues with other extensions so I hope F-Secure will sort the problem ASAP. At the very least, they need to stop saying the extension can be used until it actually can be. At the moment TalkTalk Supersafe Preferences states "Browsing protection features need a brower extension to work properly. You can install the browser extension for your current browser here." and below that a button that states "Install browser extension". Except if you follow that it downloads then won't work because Safari blocks it. I don't have any other browsers installed so it's just wrong stating the extension works with the current browser since it clearly doesn't.
Some people, like myself, have tried other browsers but prefer Safari for various reasons.
Looks like F-Secure are on it: https://community.f-secure.com/t5/Home-Security/FS-Protection-for-Mac-build/m-p/110704
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I've just installed Mojave and can confirm the problem still exists. I didn't actually expect the change to Mojave to solve anything given the problem is related to Safari 12, not Sierra, High Sierra, Mojave or any other version of MacOS.
Two weeks ago F-Secure were still reporting that browsing protection is not yet supported in the Safari 12 browser. And saying "This is a known issue, and our development team is working on the solution. The fix should be ready in SAFE's next release (17.4)"... (on which SuperSafe Boost is based)... "Although there is no fixed date for the next SAFE release, you can expect it in October 2018."
In the meantime, we recommend that you use one of the alternative browsers (Google Chrome or Mozilla Firefox) to have access to their browsing protection features.
I see that F-Secure SAFE 17.4 has been released. But the article on browsing protection remains unchanged. So, I've asked Ady for an update from F-Secure.