The Chrome extension is here. Click on the link while using Google Chrome and follow the instructions. Click on the Add to Chrome button.
Chrome will ask you to give the extension certain permissions. Here's a detailed explanation of how we use each of these permissions.
Check out this YouTube tutorial that walks you through the process of adding the hypothes.is Chrome extension--for regular annotators, the most optimal way to use Hypothesis.
And here's a step-by-step slideshow on adding the extension to your Chrome browser.
At the moment, we don't have an extension for Firefox. If you want to use Hypothesis with another browser, use the Bookmarklet.