I just checked in Firefox Aurora 14.0a2 and the Firefox Nightly 15.0a1 as well as Firefox 13.0b3. Sticky Password 6 beta works in all of them. Do the following for each version:
add boolean extensions.checkCompatibility.13.0 and set to false
Firefox Aurora 14.0a2
add boolean extensions.checkCompatibility.14.0a and set to false
Firefox Aurora 15.0a1
add boolean extensions.checkCompatibility.15.0a and set to false
This is the way around the current limitations. I have not seen an issue with doing this. Remember however using these versions means you might encounter unforeseen issues due to less testing. That said the beta feels pretty much production ready. They all feel faster than the current release 12.0 version. Enjoy.