One issue I have every time I upgrade Firefox is the loss of some of my favorite extensions due to incompatibility with the new version of the browser. Preview Release 1 is no different. Luckily I stumbled upon a link to theExtensions Mirror which is much more up to date than the standard Mozilla extension site. Many of the extensions that are unavailable on on update.mozilla.org, are already available on this site. And they work perfectly well.
Note: There are a couple of areas on the site where you might find the Firefox extensions you are looking for:
The same holds true for Thunderbird: