Brad had me run a purge tool, reboot, and reinstall WindowBlinds. I chose one of the default skins and rebooted again for good measure; didn't change any other settings. It didn't change the results—Firefox Developer Edition and Waterfox both still crash immediately.
I ran the SD Support Tool and sent the results to Brad in the hope that it will assist in reproducing the issue.
I can make do with standard Firefox for now, since I figured out how to run two instances simultaneously (with the -no-remote command line parameter); one with my Waterfox profile and one with my Developer Edition profile. Still would really like to see this fixed soon.