The easiest way to solve this problem is to do a Reset of Firefox. The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information.
Note: This will cause you to lose any Extensions and some Preferences.

• Open websites will not be saved in Firefox versions lower than 25.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

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Thank you.

I need to set my browser to work as per my convenient and preferred way. I had set the settings as per that itself. But sometimes the settings get restored to the firefox default. I need to change that. You have answered me how to reset the firefox settings. I am not asking for that. I need to disable the frequent change of my browser settings. Please guide me to do that.

What settings are you talking about?

First of all I would like to thank you as you are spending your valuable time to answer my questions. I had set my firefox settings in such a way that the history is not enabled and the auto update of firefox is turned off. There are the main settings which I like to set permanent for my browser. There are some more settings which were also set as per my convenient way. These setting get changed automatically without my interaction. So I need to change the frequent change of my browser and instead of that i need my browser to work as per the settings I am preferring. Please help to do this.

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save