It’s possible to post from the mobile site. I do it often from mobile Safari, Firefox, etc on iOS.

In desktop Firefox you can fake the user agent on a per-domain basis. I do so to pretend to be on Android when visiting Instagram.com

The only feature that doesn't work is hold-to-slide a photo around in the editor.

The webapp now lets you post things, cant edit as much as you can in the native app but has basic filters etc