Pasteboard handling is certainly app-specific.
You should be able to inspect what pasteboard contents the two different browsers are creating by using a macro like this:
Applications typically create more than one kind of clipboard content when they copy, and it's up to each pasting applications to select which of the available pasteboard formats they use.
If you find that one of Firefox's clipboard elements works better than others, and Notion is making a choice which doesn't work well for you, then it may be possible to write a script (for a KM Execute JavaScript for Automation action, which intervenes by pruning out the unwanted format from the pasteboard.
(public.html vs public.rtf pasteboards, for example)
Taking a quick look, the Firefox clipboard does look a bit impoverished when we copy from the page you link to. Unlike Safari, for example, it doesn't create a public.rtf pasteboard, and the style of its its public.html clipboard is quite puritan and reduced.
Simplest approach may be to use a different browser ?