It's confusing. The present tense "very dark fir" built-in doesn't match your house's woodwork, and you need to stain it ... lighter? You can't stain something to lighten its color.
I would try to remove the haze with acetone and steel wool. Maybe a positive-acting stripper like Kutzit. Failing that, sand or scrape off the surface to new wood.
Casey

Thanks for the response and apologies for the confusion. The 'very dark fir' of the buffet is due to the aged shellac only. Alcohol brought it down to bare wood; it had never been stained. But the shellac is not salvageable so in order to get it back to a dark color once I get it off, I will have to stain it. Thanks again, I will try the acetone.

There are a number of grades of shellac, but some are only commonly available as flakes.

Shellac does not typically darken with age, so yours may have been 'Garnet' grade or even darker.

A quick thought; test a small area with the new shellac finish, if the silvering disappears, it will also disappear with stain. Then remove the test patch of shellac and go for it. As brick suggests, a darker color of shellac may do the trick.
Here's a sampling of ultra blond (left) and garnet (right) shellacs; the woods are T to B: yellow pine, maple, white oak, and mahogany.

Kusumi Seedlac is pretty dark as well as button lac. I think you can buy shellac flakes at shellac.net. It is really easy to use and apply. Shellac has become my finish of choice these days.

You will get the white haze if you are stripping shellac with denatured alcohol and the humidity is high. That has been our experience.