Firefox: clicking on Avast toolbar icon shows empty window

Hi,

I have Firefox 32-bit v51.0.1 and Avast 17.1.2286 on 64-bit Windows 10 (v 1607, build 14393.693).

After I got the latest Avast program update, there is a grayish Avast icon on the FF toolbar in the upper right where the addon icons are. Clicking this icon opens a completely empty window over the FF window. Shouldn’t something be shown in this window? What should it be? If this is broken, how can I fix it?

Thanks.

Doug