If you are currently hosting a Sync 1.1 server I strongly encourage you to migrate to 1.5, we removed the legacy parts of Sync in https://bugzilla.mozilla.org/show_bug.cgi?id=1296767 which means it will not work anymore on Firefox 54.

I'll investigate that, thanks. However, the last time I checked I'd have to set up and run a separate account server service as well and since weave still works I had no reason to invest the effort into switching.

Regardless, I'll likely be switching to FF 52 ESR so the point will be moot.

Try connecting to it with a new profile on a recent Firefox without jumping through hoops.

Yeah, I know the process isn't exactly straightforward.