Note: I believe "un-fork" in this context means the Tor browser being able to use the latest version of Firefox (instead of an old fork), by Mozilla merging-in some of the privacy enhancements made in the Tor Browser, for ordinary clearnet use (likely as default-off options). It does not mean making Firefox into the Tor Browser. It's a great way to make the efforts of both teams available to both groups of users. I'm…
"Un-fork" in this context means "upstream", or get the patches that Tor browser uses into mainstream Firefox.
The developers of Tor Browser will still always need to check that Firefox isn't doing anything improper and that privacy is maintained – but upstreaming the patches will make it a lot easier.