There were no changes that would make WebRep break and there is a fixed release schedule for Firefox, so there was enough time to release an WebRep update BEFORE v6 was released (only needed change was to bump up the compatibility flag in install.rdf, a matter of 5 seconds of work).
The new Firefox release cycle doesn't allow any outward facing API/ABI changes in the beta stage, so there are always at least 6 weeks to test and (if needed) update the extension, no need to wait for the actual release.
That said, just now I'm posting this using Firefox 9, and WebRep is still working.