Just wanted to pass something on that was quite interesting. FBWM and Firefox Sync do not play well together. You should disable tabs and history in FF sync to keep everything working properly. Here is an email from Mike Connor of Mozilla explaining this.
Actually, tab sync wasn't the problem, history sync was, but you're still almost certainly right! If the URLs are unique, and opened in tabs, there would be a history entry created for each URL. We actually had around 250000 history items present in your account, which is an incredibly high number. (The average number was around 1500 last time we did in-depth stats.)
To see if this is true, there's an add-on written by a member of the Firefox team that can pull stats from the history/bookmarks database: https://addons.mozilla.org/firefox/addon/places-maintenance/
Once that's installed, go to the add-on manager and go to the preferences, check the stats box, and copy/paste the results into an email. The key line will look like this: "Table moz_places has 29254 records" but I'm actually curious about the full set of stats. My local DB is capped at around 100k entries (it's tied to system memory), so I'm curious how your local DB looks!
Here is what I had with this.
History can store a maximum of 104858 unique pages Table moz_places has 510955 records Table moz_historyvisits has 528036 records Table
Hope this helps!