The TIFF viewer extension will intercept a number of different requests for tiff images, capture the data and render it appropriately in the browser.
By hooking into the browser request we can detect most requests that would load a tiff image. Normally the browser would abort such requests since it cannot render the format. Instead we take over and load the image data. We can then use an in memory canvas element to render the image with the help of libtiff.js, a javascript port of the C TIFF library. The canvas element then gives us a image URL containing the decoded data as a PNG image that we can redirect to instead.
The end result is that tiff images are intercepted, turned into PNG images and end up rendering just fine in the browser.