First of all, please go to the 'Extensions'/'Add-Ons' section of your browser settings and look for ZenMate (see pictures below).
If you cannot find ZenMate in the list, it means the extension isn't installed on your browser. In this case, please reinstall it via zenmate.com/products.
If you find the extension in the list, it is installed on your browser and you can easily get the icon back by following the instructions below.
In your browser's Extension settings you should be able to click on "Show icon" next to ZenMate, which will make the icon reappear in the toolbar.
Please click on the three stripes on the right-hand of your browser. There you will find the "+ Customize" Firefox option. When clicking on it, you should be able to see the ZenMate icon displayed on the left side. Now you can use your mouse to drag and drop the ZenMate icon to your browser toolbar (see picture below).
If you run into any problems, please contact Support and we will help you investigate further.