"Suspicious": totally agree.
"Ridiculous": not really. The add-on interacts with the page content to find the text you want to highlight. And it needs to be able to do it for any site. That's what "accessing data" is about. Unfortunately Firefox developers didn't find a more specific way to alert users.
You can read https://support.mozilla.org/en-US/kb/permission-request-messages-firefox-extensions for more information.
But in any case, if you don't feel confident in what you are about to install, it is generally best to not install it.