Hi,
If you want to disable the warning when closing other tabs, set a preference to false:
In the Location bar, type "about:config" and press Enter.
The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise! to continue to the about:config page.
In the Search field box at the top, type browser.tabs.warnOnCloseOtherTabs.
Double-click the browser.tabs.warnOnCloseOtherTabs preference to set its value to false.
Warn me when opening multiple tabs might slow down Firefox: When you open a large number of tabs at once, Firefox will ask you to confirm your choice. This prevents you from accidentally slowing down your system while the pages are loading. Uncheck this option to disable this warning.
Don't load tabs until selected: Firefox will only load the active tab when it starts or restores your previous session. This can make startup much faster if you are restoring many tabs. The other open tabs will be loaded as you click on them.
When I open a link in a new tab, switch to it immediately: When you middle-click on a Web link (or hold down Ctrl while clicking with the left mouse button), the page will be opened in a new tab. That page will not be displayed and will load in a background tab. Check this option to load and display the page in a new foreground tab instead.
Show tab previews in the Windows taskbar: When you move your pointer over the Firefox icon in the Windows 7 taskbar, Windows will show you a preview of the Firefox window. With this option check marked, Windows will show you a separate preview for each tab.

Regards Deepak Krishnan P.R

Deepak's tips don't work for me. I'm running FF25 on LinuxMint.

I had this problem too, after doing a clean install of Firefox 26. Deepak's tip about configuring it through about:config rather than through Options > Tabs worked for me.

This value is set to false in ny installation, but FireFox 26 still warns about closing.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

FireFox asks the same question even when all add-ons are disabled.

But I discovered something new: If I mark the "Don't ask" button and click "Save and quit" the next time, FireFox works as expected. I didn't click that button because I didn't want FireFox to start next time with the same tabs open!

This is evidently a misunderstanding that could have been avoided by using a slightly different text in the dialog box.

Just to add that I tried opening Firefox in Safe mode and it made no difference; setting that option through Options > Tabs still had no effect - the warnings still appeared.

Yes, safe mode did not make any difference for me either.