They absolutely are, there are only slight differences in APIs. It was the entire point of killing their old APIs and starting from scratch. You can upload a .

They absolutely are, there are only slight differences in APIs. It was the entire point of killing their old APIs and starting from scratch. You can upload a .crx (Chrome's extension format) to https://www.extensiontest.com/ https://www.extensiontest.com/ to check its compatibility with Firefox, most of the time it will be (or require very minimal changes). The distribution is not compatible. Extension developers still have to upload and support their extension in a completely different store, which is where Firefox deviates from other Chromium-based browsers.