After automatic update of Firefox from v. 19.0.1 to 19.0.2 profiles.ini was either deleted or overwritten and a new profile created.
Have to admit I got a bit scared at first, but luckily all the old profile folders was present and a manual update of profiles.ini fixed it.
My questions are if
How did you installed your Firefox in ubuntu like PPA or in OPT
Create new profile and solve it.
Problem may be happen because of your preferences.
@iamjayakumars: no PPA or OPT. Pure default. Has also gone trough at least 15 other upgrades on same system.
As I stated the problem is solved as in it is up and running again with old profiles, and "create new profile" was a side-effect not a solution.
No preference settings that should cause this.