Overall, Sync leaves a lot to be desired - IMO. But you need to keep in mind that it wasn't intended to be a "cloud" service and serve to "backup" user data. It was created to synchronize specific Firefox user data between multiple devices to allow the user to move between Firefox on different "connected devices" fairly seamlessly. As far as bookmarks and "tabs" sync'd from other devices it seems to do the job quite well for most users.

It took Mozilla way too long to come up with certain features - the lack of sync for add-ons was a "craw in my gut" for many years - promised since the start of v 1.1 or even with the Weave extension, but never seemed to appear for testing in the Nightly version for Alpha testing. Meanwhile the developers of AdblockPlus and NoScript "played with" Sync for their extensions; ABP came up with "filtersets" that were automatically updated every few days and NoScript came up with a crude way the "sync" via a "NoScript bookmark" that each user could copy into NoScript prefs; to satisfy their users needs without waiting for Mozilla to finally get around to adding that feature to Sync. And when sync for add-ons eventually came about it was an extreme disappointment - the most important part of that feature was missing, synchronization of user set prefs for all add-ons. No clear documentation for add-on developers or add-on users about how the add-on sync can be enabled or used effectively. Now at best the user can go in an toggle a separate pref for each extension - but those individual prefs aren't documented. And the ultimate fallacy is that Sync isn't "synchronizing" the user's extensions, it is downloading the latest version of the extension from AMO - so extensions from 3rd party sites are ignored. At best it is a half-assed feature that wasn't worth the 3 year wait for it to appear.

As far that "Session expired. Sign in to continue." window, I have never seen that - please provide a screenshot. Sync is intended to run without any user intervention - IOW a hidden service that requires no user intervention or user action to 'work'.

As far as 'when' Firefox looks to Firefox Accounts after Firefox is opened, I think the 'timing' is 90 seconds after Firefox is launched - when you have more than one device connected to the Sync account. With only one device, I think that delay is 600 seconds / 10 minutes. note - I am not a developer or programmer, and I can't verify those prefs or settings right now as a user. I am on a new PC with only one Sync user on a new Sync account - my older PC barely works (10 minutes for WinXP to boot to Desktop) and I managed to b0rk my previous Sync account helping another user back in June.

As far as "user control" goes, in Sync 1.0 & 1.1 there was a pref that could be added in about:config to disable the "auto connect" feature and allow the user to use Sync manually using the "Sync" toolbar button. When Sync 1.5 came about with Firefox 29 that "auto connect" disable pref no longer worked. Now AFAIK you're either "Connected" or you're "Disconnected" = Options -> Sync tab - that's the only way the user can disable Sync. And the "Manage" button doesn't appear there if you Disconnected from Sync, you have to use the Sign In link that takes you to the FirefoxAccounts online portal.

The Sync toolbar button is still present and it still allows the user to force a Sync manually and the tooltip on hover still displays the D-O-W & Time of the last Sync connection, but it can no longer be used to turn Sync on as with Firefox 28 and earlier versions of Firefox (4.0 thru 28.0 was Sync 1.1, with the original Weave extension being version 1.0).

Sorry about the length of this missive, but Sync is a real sore point with me. I used the original Weave extension back in the days of Firefox 3.5 & 3.6, and was very disappointed when Mozilla decided to build it into Firefox as Sync with Firefox 4.0. I have had no real use for it for years now, but I do keep up with the changes, more as a "hobby" than as a real user.

Thanks for the reply. Here are a couple of screenshots. The 1st screen is the sync options page and the drop-down menu showing that it appears as tho I'm signed-in to Sync. The 2nd screen is what I get when I click on the Manage button. This happens all the time - Sync disconnects on its own for no apparent reason.

That said, I have another question - is it by design that I have to sign-in to Sync every time I open Firefox? The message I get along the bottom of the browser window every time I open Firefox would suggest Sync is supposed to "connect/sign-in" automatically. Sync has never connected/signed-in automatically for me.