Uhh I'm no mathematician but 8% for firefox at 8 hours and 8% at 2 hours for sync kinda tells me sync is eating your battery but again im no mathematician and I'm also extremely blazed right now.
I honestly pop into Firefox for a couple seconds and back out. I spend way more time in Sync. The Firefox bug is known to the developers I believe, but Nightly, so...
I guess the actual usage doesn't seem bad, compared to my browser with similar active times.
The warning was for frequent wake ups which makes sense with the background usage. I set it to limit background for now, at least until push notifications are implemented, as I don't think there's any reason currently to be active in the background.