In theory, it should just have been a change under the hood - use a different browser engine. But it seems that part is way more interlocked with the overall UX of the whole browser. 90% of features were missing after the switch, and while many were reimplemented later, whenever I checked it out again it was just different. I stuck with Opera 12 for so long it wasn't even funny anymore, more and more pages broke every day, but eventually, slowly I got used to Firefox. Which really took a loooooong time, since I tried a lot of extensions and tweaks to make it feel right. Weird how strongly you can get attached to a browser and its UX. I think even switching from Windows to Linux wasn't that tough.