I've seem similar posts, and tried a few things from them, but none covered my particular scenario. I'm hoping one of you will have some ideas.
With RoboHelp 9:
Started with a version which had previously been generated and presenting properly in both IE and Firefox (saved in SVN)
Removed the word "Add" in about 5 topics.
Now the TOC isn't showing on either IE or Firefox.
Took a copy of the project file that was currently working well in production.
Opened it in RoboHelp9 and made no changes at all.
Regenerated and now the TOC is also missing in IE and Firefox.
Since this problem was occurring and we had struggled with many unrelated problems in the past, support services thought perhaps the best thing to do was uninstall/reinstall RoboHelp 9. They then discovered they no longer had a copy of RoboHelp 9 and upgraded me to RoboHelp 11.
With RoboHelp 11:
Converted the project file and repeated steps 1 - 3 above with the clean copy saved in SVN.
Now, the TOC shows in IE but not Firefox.
I noticed when I launched the index.htm in IE, I got the activex error that I used to get in the past but had stopped getting after selected "Allow blocked content." I select again to "Allow blocked content" and I was able to see and use the TOC on IE.
When I tried to access the same in Firefox, I get no warning, but I still have an empty TOC.
The .HHC file is there in my WebHelp directory and I use the same source for IE and Firefox.