What it does

Click the toolbar icon on any Zillow or Redfin listing to extract the asking price, price history, and HOA fee and open them pre-filled in the full calculator.

Rent vs. BuyTerminal wealth comparison, inflation-adjusted
Historical CAGRAnchored to real sale history from the listing
Monte CarloP10/P50/P90 fan chart across thousands of scenarios
Rental AnalysisCap rate, cash-on-cash, break-even price
Option A — Firefox Developer Edition

One-click install

If you have Firefox Developer Edition with xpinstall.signatures.required set to false in about:config, click below to install directly.

Install Extension (.xpi)

Dev Edition only — regular Firefox requires a Mozilla-signed extension for permanent install.

Option B — Any Firefox

Temporary load (any Firefox)

Works on standard Firefox without Developer Edition. The extension reloads each time Firefox starts — good for trying it out.

  1. Download the extension and unzip it into a folder on your computer.
  2. Open about:debugging in Firefox.
  3. Click This Firefox in the left sidebar.
  4. Click Load Temporary Add-on… and select the manifest.json file inside the unzipped folder.
  5. The extension icon appears in your toolbar. Navigate to a listing and click it.

Open the calculator without the extension

The full calculator also runs as a standalone web page — no extension required. You'll enter property details manually.

Open Housing Investment Calculator →