Extensions create extra functionality for Mobirise. This could be anything; a theme, a block, a Code Editor, anything. Mobirise has created a whole bunch of extensions, some of which are free. The Mobirise extensions can be downloaded and installed from within Mobirise.

It's also possible to install third party extensions. Those have to be downloaded manually. They can be installed through the “import” function of Mobirise. Third party extensions have the mbrext filename extension.