Short answer: I’d recommend a narrow, lightweight, UV-resistant resin railing planter box—roughly 24 inches long—with adjustable steel brackets, drainage holes, and a removable drip tray. A model like the HC Companies 24-inch railing planter or a similar Bloem railing planter is a practical choice. It holds enough potting mix for basil, chives, thyme, oregano, and parsley without becoming too heavy, and the railing mount keeps the floor of the fire escape clearer than a freestanding box.
That recommendation comes with an important caveat: a fire escape is first an emergency exit. Check your local fire code, building rules, and lease before adding anything. Never place planters on steps, landings, ladders, or anywhere that narrows the path. If your fire escape must remain completely clear, don’t use it for herbs; choose a windowsill, balcony rail, or indoor sunny spot instead.
If planters are allowed, mount the box securely to the outside of the railing with metal brackets or heavy-duty zip ties, and use a lightweight potting mix with perlite rather than garden soil. Choose compact herbs and avoid tall, top-heavy plants that catch wind. Water carefully so runoff does not drip onto neighbors or freeze on the stairs. In winter, bring containers inside or empty them so they don’t become brittle or icy hazards.
If railing mounting is not permitted, small fabric grow bags in a sturdy tray can work as a lower-profile alternative, but only if they sit fully out of the walking path and are secured against wind. Avoid heavy ceramic, long wooden troughs, and oversized self-watering boxes on a fire escape. For most urban herb growers, the best planter is a compact resin railing box with secure brackets, good drainage, and a clear safety-first setup.