Install and try it out!
Also check out the options available. Go to the Tools -> Extensions menu, click on the Tab Preview entry, and click the Options button.
Please note that if you build your own Firefox from source, you'll need to have --enable-canvas in your build configuration. You can check to see if this is the case by typing about:buildconfig into the URL bar. If you see an empty gray box instead of a preview, this is probably the case. (Gentoo users can reportedly add the "mozsvg" USE flag.)