Last edited Sun Sep 15, 2013, 11:33 PM - Edit history (1)
Although it should work as a lot of keyboard hotkeys are common between applications.
If you know where you saved your exported FireFox bookmarks.html file, proceed to opening that file in Opera in the next paragraph. Otherwise you should probably just export your FireFox bookmarks again so you know where to find it. In FireFox, hit Ctrl+Shift+B to open the Bookmarks window. Along the top there should be an Import and Backup button, click that and choose Export bookmarks to HTML.... Save the file where it's easy to remember where it is (e.g. the Desktop).
In Opera, hit Ctrl+O to bring up the open dialog, navigate to your FireFox bookmarks.html file and click the Open button. You should now see a list of your FireFox bookmarks as text links. You should be able to right-click each link and select Bookmark Link.... You may have to highlight the text, Ctrl+C to copy, left click off the text, then right-click the link to bookmark and Ctrl+V to paste in the Name for the bookmark.
Before spending the time to right-click all of those - I'd try importing them again in Opera. I'd try creating a new folder to put your FireFox bookmarks.html file in so it is the only thing in there and then hit Ctrl+Shift+B to open the Bookmarks window in Opera and see if you can Import Firefox Bookmarks... under the File button there (if that's what it's called in the new version).
If that's not how it looks or works in the newer version of Opera, you could try this Bookmarks Manager extension...
https://addons.opera.com/en/extensions/details/bookmarks-manager/
... and point it to your FireFox bookmarks.html file.