One thing to try:
- click the 3-bars Firefox icon at the top right
- click Customize
- if you see the VDH icon somewhere, drag it back to the toolbar, click Exit Customize, you are all set
- click Restore Defaults at the bottom
- if you are still missing the icon, restart Firefox
If this doesn't work:
- open URL about:config
- search for parameter browser.uiCustomization.state
- right click on it, pick Reset
- restart Firefox if necessary
If still having the issue, make sure you are not running Firefox 46 or 47 which is known to have compatibility issues
If still still in trouble, ... well ... to be honest, i don't know what's going on