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.
The extension withExEditor[1] can open the editor of your choice in its own window, optionally with a different editor config file. Your editor writes to a file in /tmp and on each write, the text in the input field or area in your browser is updated with the contents of the file. This is an advantage over Tridactyl, by the way, since in Tridactyl you have to close the editor in order to update the text inside the browser.
While the effort for creating an overlay that's painted on top of the original textbox is well intended, a simpler approach would be to open a fixed size window on a fixed position that the user can configure. This way you could set it to your preference and it would work for all kinds of box sizes.
Or maybe give the option to only fall back to this fixed size, fixed position textbox if the box dimensions are smaller than a predefined size.
Yes, that's why I tried tridactyl instead in the end because I was tired to configure firenvim to look and behave like usefully. It combines vimium and firenvim in just one extension and supports having a vim-like config file in your home directory.
The documentation[1] shows examples of how to wrap configuration code and if then block so that section will only run when being triggered from the browser. You can also use this same technique to make config from your terminal that doesn't fit in the extension not appear.
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.