hi, this is an issue with synchronising your history. it's likely that you have a single entry there with a humongous url. maybe you're able to find this entry in your history and to delete it selectively in order to get sync working again. other alternatives would be clearing the whole browsing history or if you don't depend on it to exempt it from getting synced in the firefox menu > options > sync panel.

Thanks for your help! I found 223 very long URLs from a particular web site (HealthNet.com) in my history. Each was between 28k and 29k in length. I deleted them and sync now works. I plan to report this as a bug, since Firefox history sync should accept any URL that Firefox itself can use.