As Epoc points out in a comment above; Twitter, Pinterest and some other sites are all early adopters of Apple's new pinned themed favicons. Unfortunately the new favicons hijack the existing favicon syntax which causes conflicts in Firefox as the sites are not declaring the multiple favicons in the correct order.
For more detail see the WHATWG email thread https://readable-email.org/list/whatwg/topic/apple-s-new-link-rel-icon-mask-not-quite-favicon-syntax-causing-problems-in-other-browsers-needs-standardization
Yes, the black favicon is an SVG file, although SVG files don't need to be black (or even single-colored) so I'm not sure why they didn't just use their standard light blue there too unless they specifically want people to notice it.
Incidentally, Pinterest is doing the exact same thing right now too—black SVG favicon for Firefox.
These are their declarations:
<link rel="icon" sizes="any" mask href="https://abs.twimg.com/a/1433192681/img/t1/favicon.svg"> <link rel="icon" sizes="any" mask href="https://s-passets-cache-ak0.pinimg.com/webapp/style/app/common/images/pinterest_badge.ccdac53e.svg">