This is pretty good and does what I want. You can open a a markdown (.md) file on your machine in Firefox. There are multiple ways to style the page.

By default a small enlargable box is top left and you can click on this to get 3 options.

The first allows you to switch between the default and GitHub markdown style. (no utf-8 on one?).

The second has a decent list of styles that would be good but this didn't work for me.

The final option is a button to download the file as HTML. This works though the HTML is rather messy as it has all the code and styling for the dropdown box. You can remove the dropdown box though in the app’s settings though then of course you can’t click it to save as HTML. However you can delete the excess code from the file if you want to, though it's an extra step.

At least three other ways exist to to style pages. One is to write custom styles in the text box on the apps settings page. This is good for default styles.

Another way is to link to an external CSS file (put the link at the bottom of the page otherwise it won't work properly). You can also use an embedded stylesheet anywhere on the page (between two style tags).

All in all this is a great little add-on. Loses a mark from me because the dropdown styles didn't work for some reason and the generated HTML was messy.