We could do this at least for history, bookmarks and forms which should all have short enough GUIDs. Passwords still have long UUIDs for GUIDs, so we would need to make this an engine-specific const if we were to change it.

Random thought: the best batch size might differ between EDGE, 3G, 4G, and WiFi, both in terms of waiting around for data vs processing it, and in system resources used for the connection and other tasks. It's probably worth experimenting to see if this hypothesis is correct, and perhaps even dynamically tuning batch size based on the available connectivity. (Not to mention that if I'm on EDGE, I'm going to need my bandwidth for the task for which I have the browser open, not necessarily for syncing -- "sync on idle" comes to the fore here.)

Sadly Fennec does not expose this information (bug 531874). Would be nice for sure to do this, but I also don't think EDGE is going to be a big limit here. You're unlikely to do your first sync over a slow connection and after that we just sync deltas which isn't going to be much data. My 312 form records for instance occupy just 115 KB. Even if we fetch that in 4 in instead of 7 batches it's probably not going to be a big drain on your network.