Maybe your security (Anti-virus) software is checking files while Firefox is starting and takes so much time that Firefox can't access them and decides to skip loading those locked files. If that happens with sqlite database files like places.sqlite then bookmarks won't work properly. A second start works because the AV software had already checked the file. So you need to check the rules for Firefox in your security software.

A possible cause is security software (firewall) that blocks or restricts Firefox possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox.

See Server Not Found - Troubleshoot connection problems and Configure firewalls so that Firefox can access the Internet and http://kb.mozillazine.org/Firewalls

I tried changing settings to the Windows Firewall and to Avast Antivirus. No luck so far, but your advice make sense. I am also running Advance System Care and tried changing the "privacy sweep" settings off/on. No luck, but I think the advice is sound: It must be something running either when Windows boots, or when FF boots. I haven't made any recent system changes that I can remember which is why it is a bit of a mystery.

Just to make clear, the only thing that seems to be affected are the customized toolbar changes in my profile. All the addons are functional, bookmarks are intact. Some time ago (maybe a two months ago) I added a new profile thinking the old one may have been corrupt. Still have the same problem.

Well, ultimately, I just uninstalled FF 3.6.10 and reinstalled. Kind of a pain with all the addons, but it worked. Over time something got corrupted as far as I can tell.