TL;DR: Helping debug certain kinds of errors require increasing your log verbosity so we can see what’s going wrong. This explains how to adjust your Firefox config to do that.
In some cases, your default-verbosity Sync logs don’t have enough details.
Fortunately, Firefox has a lot of config options, and several of these options can help you (and, if you share your logs, us) see what’s going on when things behave badly.
In order to turn on logging for passwords and HTTP traffic, please enter “about:config” in your address bar and change these two config options to “Trace” (with a capital T) and restart Firefox:
Once that’s set up (and Firefox restarted), re-Sync (via “Sync Now” in the Tools menu) or do whatever it is that caused the error you’re trying to help us with, then following the previously-posted instructions on submitting logs to us, share your logs with us.
IMPORTANT: Note that personal information might appear in the log with these config options set to ‘Trace'; please be careful to change any sensitive data (for example, change your password to ‘xxx’ or ‘***’) so that the contents of the logs only show information you’re comfortable sharing. However, please don’t delete whole lines if you can help it.
Once you’re done reproducing & sharing your logs, you can (and should) change the config settings back to “Debug” and restart Firefox again.
Thanks for helping, and if you have any questions you can always fire up the old IRC client and visit us at irc://irc.mozilla.org/ in #sync.