This extension can read and change all data on any website I visit. While I understand that this is necessary for it to work, am I alone in being very uncomfortable installing it for something so trivial (or, perhaps, at all)?
You might be able to inspect the code of an extension, but it is less straightforward and easy compared to a simple userscript. Plus, with a userscript, there is no risk of the extension author turning malicious and updating it with malware, even if it was clean at one point.
My rule of thumb is using userscripts to modify websites' behavior, and using extensions to modify the browser's behavior. (At least that is the case for Firefox 56: Firefox Quantum and Chrome limits extensions from modifying the browser much; often they are little more than glorified userscripts.)