When it's all setup it works great though.
I would second this, it took me a while to troubleshoot this problem. If you don't use Sync, then it might be irrelevant.
I also use Tab Suspender to reduce the footprint, the 'suspended' tabs also intermittently fail to sync.
Github: https://github.com/Hau-Hau/firefox-tab-suspender
When it's all setup it works great though.