So you have the "Remember passwords for sites" disabled in Firefox and you can't understand the reason ?
Well turns our you might be in automatic private browsing mode - not the mode when you select private browsing, but a hidden mode that is set when you choose:
tools -> options -> privacy -> history
Firefox will : "Never remember history"
And in this mode you can't save passwords !
Change the mode to "use custom settings for history" + uncheck whatever you need from the options to make it not remember anything anyway :)
No go back to Tools -> options -> security
Voila - it is enabled !
PS: being in the above mode also shows the "start private browsing" and "stop private browsing" in the tools menu as disabled - so performing the fix above will fix this too.