@chuchkyschild tell me about it LoL but seriously i thought when i downloaded 51.02830.0 opera developer update that opera and flash would be like it use to be .... o yea right ! more like click bait ! ..lol o well..
Well on a hunch I deleted a couple of my saved (ones that don't vanish between sessions) already installed from add-ons. I can now keep my moon jelly start page which I enjoy looking at right now because it so very calming. Now I wonder if I have a different bug. Other wallpapers I had installed but thought I had lost (vanished) are now showing. Why would the wallpaper selection not show all I have installed?
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.