" Why do all add-ons in PC-A have a reset? For example, an add-on foxclocks was added in PC-A at beginning, lots of city were added. But after fresh setup of firefox in PC-B, the city list in this add-on will be reverted to default list."

It may take a few days for a complete initial synchronization process to take place, depending upon how much data and prefs are getting synchronized. A number of Firefox restarts on each device may speed up the process as the launch process contacts the Sync server each time Firefox is started. And it seems to me that a Firefox launch process 'does more' than when using the "Sync Now" feature in the menu system; although I don't use Sync enough or with much data to "swear by that WAG" or gut feeling gained from being a part time user since the days that Sync was being initially developed and was available as the Weave extension. That was with Firefox 3.5 and 3.6 back in 2008, 2009, & 2010. Sync as a built-in feature came with Firefox 4.0 in March 2011.

Also when it comes to Extensions and Themes (which are addons), the users own installed addons don't get synchronized, the addons are installed from the Addons.Mozilla.Org server (AMO). Thus addons data could be delayed and then become synchronized at a later time. Addons that aren't hosted at AMO don't get synchronized, although the extension data may become synchronized if the creator of the extension included that feature...

Plus you need to keep in mind that the Sync server is shared by a large number of users (10's of thousands? 100's of thousand? possibly millions of other users?? I have no idea how many per server and how large the server clusters might be ...) - so the load on the Sync server that a particular user is connected could be heavy during times when "everyone in your time zone" is online especially on weekends. That would slow down the service for everyone who is using Firefox with Sync when the load is heavy.

"Why do the theme in PC-A revert back to default? If I applied a theme on firefox of PC-A first. Then I sign in in the fresh installed firefox on PC-B, The theme of PC-A will be synced to the default theme, like the PC-B one. "

IMO, that happens when the Sync processing on the Sync server isn't yet completed. As far a "bi-direction" vs "uni-direction" (or "download"), Sync makes the user data the same on each connected device within the parameters that Sync is set up with, both on the Sync server and on the connected local devices.

Thank you, the-edmeister.

I'll like to add more details here. For Add-on behaivor here, I don't expect the setting/config will sync by Firefox sync, which will bring much data to be synced. The unexpected behvaior here is: Add-on A was installed in PC -A at 1000. And then some configuraiton were applied on Add-on A. Start a Sync and wait it's compelted. Then Firefox was fresh installed in PC-B. Sign in Firefox account and sync works fine. Add-On A would be installed. So far, that's expected and looks good. Then this Add-on A will be reset in PC-A. These configuration will lost. That's the strage behavior.

Let's talk about the theme sync behavior. Theme A is aplied in PC-A. Then sync completed. All good. Like previous, Firefox in PC-B then sign-in and sync. Even Firefox in PC-A is running, theme will be set to Default. That's unexpected. Then I apply theme A in PC-A or PC-B. this theme will be synced.

To use Firefox better, you need to know what PCs you'll use firefox with. Fresh install firefox at same time. Sign in at same time. Then Any configuration can be synced. Otherwise, any fresh installation will reset/erase the personal configuration.