Can this also be in Computers & Technology? I think it belongs both here and there.

Piano Prime. Get it here: https://addons.mozilla.org/en-US/firefox/addon/piano-prime/?src=search

I really like the concept but it needs more work. You can't actually save the audio/midi files external to firefox but the tracks are still there next time you use the addon. Still, all in all, it's a very cool concept that's almost useful. I wouldn't run it with tons of tabs open as it has the occasional lag. Best with a fresh, blank session. Anyone who likes this should leave comments for them so we can encourage a new release with the features we need (like exporting mp3 and midi files).

Here's the description from mozilla.org:
Piano Prime is a fun, and very simple to use Piano App for your Firefox browser.

This addon is a lite and very easy to use App for playing or learning Piano. You can play your favorite song with the built-in keyboard, or mouse. There is also an option to record desired song(s) and replay it at any time. In total, there are 52 keyboard keys but only 27 of them are visible when you install the App for the first time. This number can be changed from the options page. A setting is available to hide the built-in keyboard (top section). Moreover, sustain-control option makes piano bits to last a bit longer (for a more professional playing). Overall, this App is a user-friendly and very lite plugin available for your Firefox browser.

Some Important Features:
1. The App and all the settings are auto saved to the memory.
2. You can play piano with the keyboard, mouse, or the built-in keyboard at the top.
3. Number of visible keys are adjustable to your desired choice (default is 27).
4. You can hide the keyboard from the options page.
5. There is also a sustain control key to match piano sounds to your need (default is off).
6. You can record or replay any song(s) from the sidebar settings panel.