When you download your browser extension: https://metamask.io/download.html please check your extensions. Depending on the browser you are on:
You will see your extensions under this icon. Clicking on it will bring down the dropdown where you can pin your MetaMask extension:
You can find details about your extension in the Manage Extensions section: chrome://extensions/ or clicking the 3 dots menu on the right next to the extension you would like to manage. In Manage extensions you could check your extension version, update it and remove it.
Click this menu in the browser where you will see Add-ons and Themes or navigate to about:addons.
To make your extension visible on Firefox please enable it by switching the tab slider: