That is OK.

If "Remember History" is selected then that means that all history and cookie settings have the default value.
"Use custom settings for history" reverts to "(Never) Remember history" if all History and Cookies settings have the default setting.
You see "Use custom settings for history" if at least one of the History or Cookie settings is not the default to make you aware that changes from the defaults are made.

Thank you for your reply and explanation, but how do I remove default setting from either History or Cookie?

Why aren't you satisfied with the default history settings that you currently have?

Any change (e.g. disable third-party cookies or not remembering form data or keep cookies until you close Firefox and make an exception for cookies that you want to keep) that you make will cause "Use custom settings ..." to stay selected if you really want to see it.

It's not that I'm not satisfied with the current settings, it's just that I'm one of those curious people - wanting to know why suddenly I couldn't change my history setting when I could always chop and change it before to a setting of my choice. I noticed this has happened since recently updating to Firefox 6. When things like this just happen I always search for a reason. Anyway, it sounds as though I am a bit of a novice on Firefox compared to yourself, and if you recommend that it's probably best to stay on Remember History and it's not likely to cause any problems, then say no more, I shall accept your advice. Any more tips would be greatly appreciated! Thanks.

Hi cor-el, it's me again. I've just been playing around with the history settings and see what you mean now. I unchecked 'accept cookies from sites' and the use custom settings etc. saved. I'm back on Remember History now, but I just like to know how these things work. I learn a lot from messing around and today I learnt how to block those annoying ads by downloading Adblock Plus to Add-ons - great stuff, it even cuts out the ads when I'm catching up on missed TV programmes. No doubt there will be other mysteries I'll need to solve, so I'll always ask for you.....that's if you don't mind!