Do the following in Sigil:
Search for:
replace it with:
Code:
<hr class="sigilChapterBreak" /><h3
Then select
Split at Chapter Markers from the
Edit menu and regenerate the TOC.
BTW, the ePub also contains a couple of syntax errors. Click the green check mark to search for them.
Most readers have limited memory and cannot open ePubs that consist of single large page. It's best to test ePubs with
ADE instead of the FF extension, which is more forgiving.