If you use a master password, Weave Sync will automatically connect after you enter in your master password. Weave Sync will stay disconnected until you enter your master password or you choose to manually connect.It drove me crazy to have the request for the master password come up every time I started Firefox, and it totally baffled anyone else using my computer (you can just cancel, but they would think they couldn't use Firefox without the password)
I had actually disabled Weave on most of my computers because of this. But that was a pain because then my passwords weren't synced. Now I can re-enable it.
Now if only it would just sync my add-ons and work with my iPhone ...(users are never satisfied!)