I browsed to https://en.m.wikipedia.org/wiki/USB, a page I had not visited before. I scrolled down to the TOC, opened (expanded) it, and clicked on "History | Version history | USB 3.1", a level-4 section. The entry became underlined, and it changed color from blue to purple, indicating that the link had been visited, but nothing else happened.
I scrolled down to the (closed) History heading and clicked it; it expanded. I scrolled back up to the TOC and clicked on USB 3.1 again, and the screen went to the subsection, which was open.
**If the TOC shows a subhead, that section should be accessible.** Clicking on it provides enough information for the system to open the containing section(s) if necessary, and the targeted subsection, and then to display the subsection.
I am using a Samsung Verizon Android Galaxy S-3, SCH-I535, OS v4.4.2, with Firefox 37.0.1.