This has happened to be more then once. For some reason it seems like from time to time a client figures that it can't fetch a add-on and then it seems to tell every other Firefox client to just uninstall it.. problem is when this is all your add-ons.
First check if you have a log over what actually happened. Enter "about:sync-log" this will show you the weave log directory. The last log should contain the information over what happened. At times there are no actual errors.. so there are no logs.
What I found out is that you can see what it successfully syncs by modifying the value " services.sync.log.appender.file.logOnSuccess " in about:config from "false" to "true". You can then look at the log(se above) where you will have a "success-" instead of a "error-" logfile.