I gave firenvim a shot a few times in the past. The pain points for me were competing shortcuts between extensions (e.g. vimium) and too small realestate in most text area prompts where firenvim activated. I believe the messaging then was that you could just copy paste your nvim config over, and firenvim would use it in the browser. While I think that's true, what makes a good config in the terminal is not the same as what makes a good config in small text area prompts on most websites.

EDIT: I remembered another issue. I'm not able to use it at work because my employer doesn't allow extensions that access all data on a website.