IIRC, Mozilla Weave (now Firefox Sync) encrypts all of your data on your PC before uploading it to a webdav share- Either your own, or theirs.
If you use your machine, they never see it, and if you use theirs, they can't decrypt it.
Weave is about the only thing I really miss since switching to Chrome; I ran my own server, but encrypting the data prior to transmission meant I wouldn't have felt terrible about sending it to them. Architecturally, I felt pretty good about it.