Not just live on, but are essential in navigation. I have so many open tabs that the favicon is the only differentiating thing between them. When I last tried to switch to FF from Chrome the lack of pre-cached favicons made all my bookmarks look like shit and I just couldn't stand it, so I bounced back.
Just closed and reopened FF 79 and all icons are right there where I have left them.
On Firefox, you could go to the Library and right click the Bookmarks folder, choose "Open all in tabs". It'll take a moment but all your icons should be fetched afterwards.