How To Get Missing Hola Or Greasemonkey Icons Back On Toolbar

The other night after installing the Firefox Add-on Ant Video Downloader, then I realized my Hola and Greasemonkey icons which were supposed on the toolbar had gone missing.

I clicked on the far right end of the toolbar to open the menu, yet I still couldn't find the missing icons.

How did I got those missing icons back on the toolbar?

Just restart my computer.

Then when I opened my Firefox browser, the missing icons are back on the toolbar.