So, gryzor, the script has been updated!
Now it exports all folders, even empty ones. Importing the output file has been tested with chrome. Also, I don't have imported bookmarks on Opera so I couldn't test that, but bookmarks bar are exported, even empty folders, so I guess exporting everything you have on Opera 25 developer bookmarks will work just fine, even imported bookmarks (importing import export exported imported exporting).
As previously said, you need php-cli in order to run the script. More details on the script itself.
Hope you use it, it has been a while to change it, so make it worth!
THE LINK: https://gist.github.com/alesegdia/e88f3bd23a5fd83fc3a9