Got it working in FF 6.0 Beta 5 Rated 5 out of 5 stars
My rating is really a 4.5* for the current one, and assuming they'll come out with a fix soon. But through the years it's worked great for me. I went from FF 3.6 to FF 6.0 Beta 3 last week with a clean install of Win7. Tonight I finally decided to create another profile, for testing this out. I read that if you change the install.rdf and <em:maxVersion>4.* value from 4.* or 5.* to 6.* you can make it work. Well I did that and it works. Granted I made a Profile backup of 3.6 and when I installed FF 6.0b3 I just did a clean install and installed all my addons individually. But I also saved the settings (exported within the options of the addon) of all of them to start with a clean slate if possible.
Worked very well, but I missed not having ScrapBook and CNExtend (got it to work also with same method). Forgot about CookieSafe, one of my favorites, but in last 8 minutes I can't get that one to work. It shows up as compatible, but clicking on the icon does nothing as before. CookieSafe's <em:maxVersion>4.0 looks a little different.
So if you absolutely need ScrapBook, you can make it work with the new versions. I also used the left click on addon bar _Edit Before Save_ and that also seems to be working correctly. Just make sure you open up two different Win Explorer windows with last Profile you saved and I'd create a new profile to test it out. Hope that helps.