I have FF as my only browser, because it is actually the only one featuring disabling history altogether.
But now I have luckily found another workaround, which is a "breeze" compared to the above-mentioned one, namely to make a safety copy of a special folder named safebrowsing being "responsible" for this sudden change to FF's settings.
If using Windows XP, this is its location: C:\Documents and Settings\<User Name>\Local Settings\Program Data\Mozilla\Firefox\Profiles\1ajtphld.default or whatever random name your profile name is.
If using Windows 7/8, this is its location: C:\<User Name>\Local Settings\Program Data\Mozilla\Firefox\Profiles\1ajtphld.default or whatever random name your profile name is.
I saved the safety copy of the folder on a USB Flash Drive for security reasons.
And whenever this problem may re-arise, I can just copy the folder back in place, as simple as that!
HTH others as well.
Edited by Queen-Evie, 22 July 2015 - 09:02 AM.
moved from Web Browsing/Email and Other Internet Applications