The add-on underlines errors while typing. That works on almost any site, except addons.mozilla.org, Chrome Webstore and Google Docs and very few others. If it doesn't work for you, please contact me at the address given at https://languagetool.org/legal/, specifying the website where it doesn't work. Using the context menu to open a new tab is meant for static texts, not for texts you have written into a text area.