Not your fault, but that is a risk of using a Beta (before Google hijacked the term to mean "Upgrades are Planned").
Bugs like what you saw are flagged as high-importance "data loss" bugs, and FF has gotten better about avoiding those, even in pre-release builds, since the bad old days of accidentally deleting non-FF data from your home folder.
I've used the same profile on my primary computer for something like 5 years, across a migration from Windows to Ubuntu, and a few times when I've had too much time on my hands and ran minefield/betas for a month now and again.
They were unstable, but I never corrupted my profile.
Bugs like what you saw are flagged as high-importance "data loss" bugs, and FF has gotten better about avoiding those, even in pre-release builds, since the bad old days of accidentally deleting non-FF data from your home folder.