I've made up a serial terminal based off of the new Web Bluetooth API, what this means is that you are able to connect your Arduino (or any other serial device for that matter) to one of the cheap HM Bluetooth Low-Energy serial modules and talk to it from your browser.
This will run on pretty much anything that runs Chrome, a few other browsers are supported / adding support. This (https://github.com/WebBluetoothCG/web-bluetooth/blob/master/implementation-status.md) link shows all the current implementations. I've tested and it works on Android, OSX and Windows (Windows requires you to change the experimental features flag). Looks like its implemented on Firefox and Opera too.
I haven't tested it, but apparently it will work on iOS if you get the 'WebBLE' app ($2) which is just a browser with Web Bluetooth support built in.
Click here (https://github.com/hewittwill/WebBluetooth-Terminal) to check out the GitHub repo, or click here (https://hewittwill.github.io/WebBluetooth-Terminal/) to check out the hosted version.
I've only tested it with a really limited range of hardware, so would appreciate some more feedback to try and make this more awesome. I've included an example in the repo that lets you turn an LED on and off from the browser. It'll play fine with Firmata, I'm just working on the browser side implementation.
This is the first project I've uploaded publicly, looking forward to seeing what you guys manage to break!