The current Firefox UI is XUL and XBL, tons and heaps of it. Re-implementing it in e.g. HTML is not an easy task, in particular not when you also want the result to look at least somewhat OS-native (took XUL itself ages to get there, btw). You also cannot do the OS-native look in HTML alone, you need support from the engine, which means servo has a some work ahead on that front too.

So for the foreseeable future, servo is not an alternative to replace XUL/XBL anyway. mozilla said that themselves.

Using servo to render the actual web pages inside the browser, that's another matter (once servo is considered stable enough, of course). To the add-on developer, this should not really make any difference anyway, XUL add-on, SDK add-on or WebExtension alike, as most add-ons for the most part will use the regular DOM APIs which servo has to provide anyway to interact with web stuff.