The "default" behavior definitely is for the tabs to slide up into the title bar next to the orange "Firefox" button when you maximize the window, and you've discovered the problem. Now... why won't it stay changed?

When you view the preference in the about:config preferences editor, does it mention anything about being locked, or does it appear in italics?

Assuming not, then there are two ways your change might be reversed:

(1) An extension
(2) A user.js file

Investigating Extensions

You can review, disable, and sometimes configure Options or Remove extensions on the Add-ons page. Either:

• Ctrl+Shift+a
• orange Firefox button (or Tools menu) > Add-ons

In the left column, click Extensions. If it's just not clear whether any of them are relevant, you can try Firefox's Safe Mode and see whether that allows you to change the preference.

Firefox's Safe Mode is a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference?

user.js File

This is a settings file that Firefox reads only when you first start it up. It will override changes you made during your previous session. To check for and potentially edit or remove such a file, please see this article: How to fix preferences that won't save.

Any luck?

Yea Thanks , It Worked

I Just Disabled Recently Installed Add-Ons And Turned "browser.tabs.drawInTitlebar" True

The Add-On Was Settingsanity But I Really need It , So Any Alternate Way To Disable Images To Load Automatically. As There Is No Option In New Version Of Firefox.

There has been an update to that extension to fix the problem. You can download again from https://addons.mozilla.org/en-US/firefox/addon/settingsanity/ or you can use the "gear" button on the Add-ons page > Check for Updates to check all your extensions for updates.