after uninstalling "No Squint" Firefox zoom is not saved - Solution
I just wanted to post the solution to the problem described in this question https://support.mozilla.org/en-US/questions/962175 because I saw that none of the replies were helpful and the person had to restore from a backup.
In my case the source of the problem was that this option http://kb.mozillazine.org/Browser.zoom.siteSpecific was set to false.
Also commented at add-on's issue tracker https://github.com/jtackaberry/nosquint/issues/93#issuecomment-43520183