Ordinary people can use browser extensions OK on desktop, but on mobile it's a mess. Chrome for Android doesn't support extensions, and no one uses the Android browsers that do. Installing an extension for Safari on iOS requires following many unintuitive steps. I hope mobile extensions become easier to install/use with time!
The original version was a browser extension. It was very painful to maintain support for all the different types of input fields. Most large social media sites do not use standard text areas.