Your caniuse search there is catching several dvh-related features, and the one that happens to be shown at the top of the page is specifically about whether those units are supported **within the CSS Houdini Typed OM API**, and Firefox is shown as not-supporting-them-there because Firefox doesn't yet support Houdini Typed OM at all. That's tracked in https://bugzilla.mozilla.org/show_bug.cgi?id=1278697 and doesn't have any special relationship to these units in particular.
Anyway, bottom line is, these units should already work anywhere that you can use any other CSS length unit (e.g. `px`, `em`, etc.) in Firefox. Here's a trivial jsfiddle example that you can try to see for yourself: