I am surprised that Firefox freely allows access to the clipboard. I'm using Brave and there's an explicit permission for it that is disallowed by default.
More on topic, this works fine within the trusted green zone of local desktop applications. but the browser is(or should be) a high security zone. with a really tricky security policy. which to paraphrase would be "Allow passing information to the rest of the os. but only as a result of a direct user request." You don't really want to disallow the browser to script selecting anything, this is useful for editors. but you probably want to maintain a flag on that selection as to if it was done by the user or not.
Well there is my useless pedantry for the day done. My apologies and thank you for letting me get that off my chest.
Lose selection === unable to copy. And it's at all times up to source app to decide what it will allow to be copied.
But I don't recall Firefox ever being hardcore security and privacy (even though some of their techies are). For Mozilla, that's a fairly recent positioning that they're growing into.
Initially, there was competition to be the most popular browser.
But most of the history is a constant movement towards having the browser facilitate what companies wanted to do towards users (moving away from the "user agent" as an agent of the user).
In parallel, Google paying Mozilla for placement, and then possibly to keep a nominal competitor alive.
In recent years, Mozilla has been positioning itself as one of the champions for Internet freedom, and I assume that some of their people were that all along (e.g., the kind who could've gone to Google, but chose to work for much less money at Mozilla). Though I don't know how genuine that sentiment is from the top, when leadership draws huge compensation, for poor performance, while laying off techies. (Techies traditionally have lead Internet freedom, from the ground, up, and leadership might be better thought of as a humble support system for that.)
brave://settings/content/clipboard