When you download the browser extension from MetaMask's official website, please check your browser's extensions. Depending on the browser you are on, the steps are as follows:
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 by clicking the 3 dots menu on the right next to the extension you would like to manage.
Click this menu in the browser where you will see Add-ons and Themes or navigate to about:addons.
On the next screen, click on 'Extensions' on the left-hand side:
To make your extension visible on Firefox, please enable it by switching the tab slider: