Please, don't be concerned about your privacy with this addon.
You can always check its code located at Homepage link, where you won't be able to find anything in the least suspicious. All it does is search the terms and highlight them on the page; no external requests or even saving your searches on your own machine.
If you look up the 'read the text of all open tabs' permission, you'll see that it just means you allow the extension to use 'find' API, which searches for the text on the page and nothing else.
https://support.mozilla.org/en-US/kb/permission-request-messages-firefox-extensions#w_read-the-text-of-all-open-tabs
And it can only perform such logic at exactly the moment you click the extension button.
New extensions are indeed more concerned about privacy and security.

Also, thanks for the suggestions on the user interface and the description.
And sorry for such a late reply.