The Bookmarks Importer was designed to import from Opera Presto to Opera blink. That's how I got my Opera 12.15 bookmarks into Opera 20 (I believe it was) or 19. Sometimes, the Bookmarks importer is not active (it's greyed out). Here's a post by Leocg where he had some interesting thoughts on the issue and how to make it work.
And then my other thought had been to simply import the Opera 12.17 bookmarks into Chrome, then copy the bookmarks file from the profile folder in Chrome to the profile folder in Opera.
There are other approaches. And Opera is working on making the import function work better, including from third party browser. Still, you bookmarks were in an earlier version of Opera, so it should have worked.
On the other hand, since you're using Opera 25, and seem to be doing okay, you may well not want to focus more on this. I guess it depends on how many bookmarks you had, and whether you were able to move the more important ones over the old-fashioned way :. One by one . . .
I've had Firefox on my computer for years. And I know people have different opinions on this. But for me, there's just no comparison. Maybe it's look and feel, but I very much prefer the new Opera.
Best of luck.