It depends on how you downloaded. If you installed an extension directly from the Mozilla Add-ons site, its page usually will show what kind of user interface it has. If its toolbar button is missing, check the Customize dialog to see whether you find it there and can drag it to where you want it. This article has the steps for that: [[Customize Firefox controls, buttons and toolbars]]. If you downloaded it to disk, you many need to run an installer. I'm assuming you got it from a trustworthy source... click the blue download arrow on the toolbar and see whether you can find it there.