How can I reset Safari when Safari is broken?

I started up Safari and let it import history from Firefox, but now Safari is behaving very strangely. For example, many of the menu options are greyed out.

I want to reset Safari to a blank slate, but the Clear History... option isn't available.

I tried deleting ~/Library/Safari/ and ~/Library/Caches/com.apple.Safari but it didn't seem to have any effect; Safari is still broken. How can I more thoroughly reset Safari when the app itself seems broken?

This is using an Administrator account. Running macOS Mojave 10.14.5 (18F203) and Safari 12.1.1 (14607.2.6.1.1).

One of the very first things to try when Safari is "behaving poorly" is to launch it in Safe Mode.

Quit out of Safari then just hold down ⇧ Shift while you launch it. That turns off all third party extensions and often lets you do some cleanup when you have stuff installed in Safari that is resistant to being turned off.