Asking for a frind, who would prefer to use the Tor browser.
A plugin for that browser would be acceptable.
Failing that, a security oriented browser which requires a password at launch, otherwise it closes.
Additionally, the browser's data (bookmarks and history) must be encrypted on disk when not running
Must be gratis and run on Windows 10.
[Update] Thanks, @Thhomas for the comment. I ought have said that I don't want a separate Windows user, as it too much trouble.
I was considering putting he portable version into a VeraCrypt container.
A program like WinGuardPro is also an option, with the downsides that it always running in background, and also the browser's data (bookmarks and history) may not be encrypted on disk when not running (I will add that as a requirement)