If you forget the path, you can always quickly find it, by clicking on the Opera Menu (upper left of the browser), and then click on About Opera. The profile is a folder (not a file) called in Opera 22, "Opera Stable." In that profile is a bookmark file, a speed dial file, i believe, (I'm not even sure which one it is) your preferences, a folder of extensions, etc.
I think you'll be okay on this disappearing extensions issue. But it's always good when Opera is functioning well to save a copy of the profile. And update it periodically. Now I usually clear history before saving the profile so it takes up less space. Flash drives are pretty inexpensive, and often connect to the USB port of a computer. You can save the profile in a flash drive easily enough. Or to the desktop (though if you ever have to reinstall the Operating system, the desktop would be gone, so that's why saving to a flash drive or even to the cloud, if that's available, is a good idea.