You can archive them to the "Archives" folder or just make a new folder with whatever name you want and copy them there, and if you really don't want to see that either you can always "unsubscribe" the folder which will "hide" it. Otherwise, I don't know of a a way to hide a group of messages in a visible folder.
Bug 1759576 Comment 7 Edit History
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You can archive them to the "Archives" folder or just make a new folder with whatever name you want and ~~copy~~ move them there, and if you really don't want to see that either you can always "unsubscribe" the folder which will "hide" it. Otherwise, I don't know of a a way to hide a group of messages in a visible folder.