How do I keep the Opera Browser's tabs active when switching applications?

When I opened multiple pages/tabs on the Opera Mini 7.5, I pressed the home button to switch apps to see SMS, then returned to the Opera app by holding the Home button. Suddenly, my opened tabs were missing, just like opening a new tab. There is a task-killer application, but I can't remove it. This application has been installed as a default application in the Android phone. I was trying to delete some of the system apps, but there was no option saying "Delete" or "Uninstall". I also turned off all the Developer Options. How to keep the Opera Browser's tabs active when switching to another application? --- My phone info: - Model number: Device-01 - Android version: 2.3.6 - Baseband version: VENUS_BP_00.03.63.b501 - Kernel version: 2.6.38.6-perf zly @ HL120 # 1