Has anybody got any idea how to write an extension for Komodo Edit 6 in Python?
It's hard to say what you need to know, because if you have a background writing Firefox extensions, moving to Komodo will be easier. If not, I would consult a Mozilla resource like https://developer.mozilla.org/en/Building_an_Extension first to get comfortable writing extensions.
Next step, I would browse the extensions at http://community.activestate.com/addons , and look at the source of one that interests you.
If you're adding UI, you'll need to know the IDs of the elements that you want to add an overlay to. You can find that info at http://svn.openkomodo.com/openkomodo/browse/openkomodo/trunk/src/ -- most of the IDs you need will be at http://svn.openkomodo.com/openkomodo/view/openkomodo/trunk/src/chrome/komodo/content/komodo.p.xul
The DOM inspector will help you find these items as well.
There are two Komodo variants of venerable Mozilla extensions I find indispensable -- Venkman (JS debugger) and the Extension Developer's Extension. You'll find them at the addons site given above.
You mentioned that you want to write the extension in Python. It's been done, but accessing the front-end is much easier in JavaScript (and sometimes can only be done in JS). Python is fine for writing components that don't need access to the UI -- you'll find these in other extensions in the components and pylib directories.
Komodo also ships with a project templates for writing Komodo extensions. It will build a "Hello World" extension out of the box, so you can use it as a springboard for your own work. But I wouldn't skip the above steps and jump right to it.
The best place to go for further information is http://community.activestate.com/forums/komodo-extensions
Good luck, Eric