Been looking for an extension that would allow vim editing in the browser for some time but now it finally exists !
It is a sheer pleasure to use.

By defaut the font will probably appear too big. Sadly the README of the project (https://github.com/glacambre) does not mention how to solve this issue. But on the issue tracker some solutions are proposed (https://github.com/glacambre/firenvim/issues/972)

Since the extension in using neovim in the backend, you should of course have neovim installed.
The `set guifont=my_font:h10` neovim option should solve the issue most of the time. Learn about the `lines` and `colums` neovim options too.