KeeForm supports the "New Microsoft Edge" browser. Since the new browser is based on Chromium, you basically just have to enable allow extension from other stores and follow the installation instructions on the KeeForm for Chrome page.
Startup Boost is a new feature in Edge starting with version 89.
Startup boost is turned on automatically when Edge is the default web browser.
In case KeeForm does not start Edge anymore, please disable Startup Boost in the settings (edge://settings/system).