I didn't notice this right away because I rarely turn off my main computer or shut down the browser, even when not in use. But on one of my backup systems, after one of Opera's last updates, the Opera browser would start up when I powered on my computer. I don't know why Opera did this but luckily it's an easy fix.
Go into Settings: in web address bar type the following and hit enter:
opera://settings/
Or press Ctrl+F12 (some might work with Alt+P). Then scroll down to the bottom of the page. In the On Startup section just before you get to the word Advance. Click on the switch to disable Run when my computer starts option. And that's it.
The only other browser that I know that sets itself up to run automatically on system startup is the Avast Secure Browser. In this case go to:
secure://settings/
and scroll down to the On Startup section. Again toggle the switch off.
