I agree 100% with you: always tag your images.

Firefox doesn't assume sRGB for untagged images and graphs by default, only if you configure it on the advanced menu: http://gearoracle.com/guides/firefox-color-management/

The point of the test is to encourage browser developers to give users choices on how untagged page elements are color managed, and the right choice is to consider then in sRGB space. Firefox is the only browser that allows this via the gfx.color_management.mode switch. This is important because not all page graphics are images and there's no way to tag a CSS element, for example.