Sign In option is available on the Navbar on the top right corner.
Recently this appears to have disappeared when opened in Firefox.
The Filter rule causing this is ||google.com/js/plusone.js$third-party. You need to disable this filter.
In Firefox, goto Tools → Adblock Plus → Filter preferences
Under Filter subscriptions tab, select Fanboy's Social Blocking List.
On the right side there are the filters. If you do not see the filters on right, hover your mouse on the dotted line on the right till a Hand appears, then click on it.
Scroll down to ||google.com/js/plusone.js$third-party and uncheck it.
Close Adblock Plus Filter preferences.
Now open blogspot webpage or refresh an existing blogspot webpage. The Navbar should be back with the Sign In option.