I have been waiting for such an extension for a long time now, and I have finally located one that seems to work.
It is appropriately named ‘Search and Replace’.
It can search text through the various input fields on the current web page and you can replace one keyword with another.
The feature set is pretty limited right now. There is no undo option. You can only do a complete find and replace through the entire page’s content. We hope that it gets more powerful and useful with time!