Are you able to get to that page using any toolbar button, or the drop-down list from the Extensions button? [[Manage your extensions using the extensions button in the toolbar]] I assume it's this extension, and some reviews indicate problems, although it's not a storm of posts: https://addons.mozilla.org/en-US/firefox/addon/ether-metamask/reviews/ Because money is involved, I suggest trying the most official support channel the author makes available, but also checking other sites for background.