This article should help - How to fix preferences that won't save

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

@TonyE: thanks for reply... I've try to delete all prefs.js --- and back to standard pref setting. But still, can't disable spell-check.

@cor-el: thanks also... I use standard theme, not so 'big fan' of skin anyway :) --- in "safe mode" also can't disable spell-check.

Only spell-check which can not save (after re-start) on FF preference, other options are working as they should be.