[Bug] Toolbar button breaks standard toolbar expansion

[Gitoffthelawn]

In Firefox, when the user reduces the horizontal size of the window, the browser will move extension icons to an "expansion icon" (double right-chevron).

When uBlock is one of those icons, it breaks the standard functionality of that expansion icon. Once the uBlock icon is included within the expansion icon's dropdown menu, every other left-click on the expansion icon does not function.

To reproduce:
1. Add the uBlock toolbar icon to the Location Bar (navbar).
2. Reduce the horizontal width of the Firefox window until the uBlock icon disappears and the expansion icon appears.
3. Left-click on the expansion icon. If it works, try it again. It will fail every other time.

Tested on Win7-SP1 with FF v38.0.5.

I actually tested it with one of the uBlock forks, but am switching to the main project here. The issue likely exists in the main uBlock project, so this is probably the best place to report it.