Rated 5 out of 5 stars
I also wondered that FF does not start after upgrade to 53. Safe mode did, obviously silently updating this addon, so the next start worked.
BUT: It did not ask for the password on startup! Did a quick research and found there were no options for this addon (1.6.3). So I had to manually remove the update and do a clean install again. Now it works as before!
UPDATE: Now version 1.6.3 already installs on FF52. I verified with my existing profile by re-installing FF52. It first disabled StartupMaster 1.6.3 (the one for 53 only), updated to the AMO version 1.6.3 (for 52-53) and it worked. Then I did another update to FF53 and it still works.
Should be fixed now.