Every device has equal weight - save for "mobile" devices with history and bookmarks, to certain degree. There is no "priority" setting for the "source" of the shared data.

As far as "add-ons" go, plugins don't get synced as they aren't installed in Firefox. Extensions and themes don't 'really' get sync'd, they would be installed afresh from the Addons.Mozilla.org website [AMO]. As to whether the user set prefs and data for an extension is sync'd, that depends upon the individual extension and what the extension developer did to make his/her extension compatible with Sync. Although the basic prefs do get sync'd. -- And if you have extensions that aren't hosted at AMO they won't "sync" at all.

As far as preferences overall goes, some get sync'd while others don't; some are controlled by 'hidden prefs' in about:config . Bottom line is that the "basic prefs" are sync'd, while the more advanced ones aren't or need to be enabled in about:config .

As far as using a Master Password and Sync goes, I recall reading that was solved as of Firefox 38.0.5 - IOW, Firefox 29.0 thru 37.0.2 didn't allow Sync when the user was using a Master Password.

Overall, I built a new PC back in April and when I tried to duplicate my Firefox installations [yes, I have multiple unique Profiles that I use for different purposes] on the new PC by using different Sync accounts I was very disappointed with the results. I have too many extensions - like 40 plus and many differ per Profile - and every hour or two I would find the new PC working a slight bit different than I had the old PC setup due to a pref not getting carried over. After a couple of days of correcting those "missed prefs" I gave up and just copied the whole damn Profile over from the old PC to the new one. And one Profile I just created from scratch on the new PC. The old PC was on WinXP, where the new PC is running Win7 - and I had a problem with the "path to" differing - AppData vs. Roaming - with regards to one very old extension.

Bottom line and YMMV, from what you have posted I don't think you would be happy with the outcome after including additional Sync prefs in the Sync process, except for maybe bookmarks.

Also, you may want to wait a few days before proceeding, to allow other contributors to weigh-in. I have used Sync from close to Day One with the original Weave extension (12-21-2007 Introducing Weave) - the 3rd or 4th pre-release version. In those 8-plus years it gone in different direction than what my needs are, so I don't really use it for anything except to keep bookmarks and passwords synchronized between a Release version, a Beta version, a Developer/Aurora version, and a Nightly version - all basic installations on the same PC with only 2 extensions to help "ID" them in the System Tray on hover - with prefs I particularly don't want the same, I try to keep up with what Mozilla is doing with Firefox, as the upcoming versions are being developed. One PC, no smartphone, and an old Netbook that rarely goes on the internet that is running an old version of Firefox.

This is exactly the kind of information I was looking for about sync. Especially your experience with it as an advanced user of FF.

I guess I was under the mistaken impression that they had expanded the capabilities of the feature but it sounds like that may not be the case.

Thanks for the response!