With Sync there is no master/slave or primary/secondary device - nothing for the user to set or select as to which device takes precedence.

As far as standard prefs go [not prefs for extensions], you're liable to get a combination of the two installations.

As far as add-ons goes, you'll end up with whatever extensions you have on each device being installed on both devices. And only extensions that are hosted at the official Addons.Mozilla.Org website will get sync'd; they are installed fresh from AMO, not actually synchronized. None will be removed from either device.

Overall, if you now have two working Firefox installations, hold off on using Sync for Extensions and Preferences to begin with. If you have many extensions and a lot of browsing history give Sync a chance to sync bookmarks, tabs, passwords, and history before adding Extensions and Prefs to the 'mix'.

When it comes to prefs for extensions your chance of having all preferences getting sync'd is slim. Not every add-on developer has 'embraced' Sync and provided for their prefs getting sync'd. I had 44 extensions the last time [6 months ago] I tried sync'ing extensions and after wasting a few hours over a few days trying to 'fix' the prefs that didn't get sync'd as I found a pref that wan't right for me, I gave up and started from scratch with a new Profile on the 2nd device, which was a new PC and a new Firefox installation, and installed extensions from scratch & set all my prefs manually.

oh, thanks so much, your info really helps!

:)

i was getting so perplexed as to why i kept getting the error msg at new FF window bottoms about sync not working... i think i discovered it was because i had the "Clear history when Firefox closes" checkbox on in one computer and not the other, (which was in turn including the "saved passwords" ) (which was yet in in turn causing me to get signed out of sync..) and no matter how many times i reset it to unchecked and re-signed in, the next time around it was mysteriously checked again...which seemed to make the error come up everytime yet again...

If you don't mind, i now have another syncing Q, which i have searched through the FF help for but can't find an answer: how to import FF bookmarks to safari iphone? ( i learned i can't sync them, but how about import):

I learned i can import via an HTML file to my other desktop, but how to bring those bookmarks into iPhone? (i tried plugging phone into my windows desktop, but all i see are photos, and it wont let me drag or add that HTML file to my iPhone...)

your help is much appreciated!!!