I used to go to the Goodwill on Rt. 50 at Glebe, it's well run but but not very big and not going to have a lot of furniture choices. A giant thrift store that'll have furniture is the Unique (if you're familiar with Savers, it's the same chain) in Merrifield. The Clock Tower in Falls Church is worth a quick stop too since it's on the way if you go to Unique. If you wanted to check all these places out your drive is basically DC -> Goodwill on 50 -> Clock Tower -> Unique along 50. Or if you don't have time just go to Unique.

Salvation Army stores here I never found great stuff but they're here as well, there's a giant one on Little River Turnpike in Annandale. From Unique it's also not far away if you wanted to supercombo this outing, and then you could jump on 395 and back to DC.

Craigslist and Nextdoor work pretty well here. Outdated furniture can be had for nothing.

People are saying Miss Pixies. I live nearby and it's a great store and not expensive, but it's not a thrift store with $50 sofas. However I do recommend it, and also GoodWood for secondhand/vintage furniture when you're in the neighborhood.