I wanted to use a proper editor to edit the text in textareas, and was
really happy to find about
https://addons.mozilla.org/en-Us/firefox/addon/firenvim/ , which embeds
an nvim instance in any textarea in your browser, including of course
the edit field for tiddlers in tiddlywiki. Since nvim is quite
extensible, that fills my needs, although I admit vim/nvim is not
everybody's text editor of choice.
Another approach is
https://addons.mozilla.org/en-Us/firefox/addon/ghosttext/ , which will
let you edit the contents of textareas in an external text editor of
your choice; I've also used that one, and I like it, but I prefer the
"embedded" approach of firenvim better.