This is caused sometimes when Firefox crashes. To fix open up terminal and type in:
find /pathtouserlibrary -name ".parentlock" -print
It should print out a list of .parentlock files that are keeping the profiles from being opened. If that list looks okay – go ahead and combine the find command with rm below:
find /pathtouserlibrary -name ".parentlock" -print -exec rm '{}' \;
In both cases replace /pathtouserlibrary with the path to the User's Library – usually ~/Library

