Sorry, the SQL query got reformatted, the correct form is:

SELECT "[" || c1title || "](**" || c0url || "**)" FROM stash_fts_content JOIN stash ON stash_fts_content.docid = stash.uid ORDER BY stash.last_visit_date DESC

I imported bookmarks to Firefox this way:
(In Opera 24 all bookmarks are saved in stash.db. I didn't find anything in bookamrks.db nor favourites.db)

• Download and install SQLite Browser
• File -> Open Database: %appdata%\Opera Software\Opera Stable\stash.db
• Swich to tab Execute SQL
• Enter command:

SELECT '[" || c1title || "](\*\*' || c0url || "\*\*)"
  FROM stash_fts_content
  JOIN stash
    ON stash_fts_content.docid = stash.uid
  ORDER BY stash.last_visit_date DESC

• Copy&paste the results to notepad
• Edit -> Replace " to nothing (remove quotes)
• Edit -> Replace ** to " (convert ** to quotes)
• Now save the file as blabla.html, don't forget to use UTF-8 to preserve accented characters, if any.
• In Firefox: Show All Bookmarks (Ctrl-Shift-B) -> Import -> Import from HTML
• Browse for the blabla.html

The bookmarks get imported to Bookmarks Menu, sorted the recently visited first, keeping the Titles and URLs.

Restarting works, same as closing down the browser and opening fresh but I'd like to keep those tabs open and not lose them all the time.
If you set Opera to "Continue where I left off", those page aren't opened when Opera restarts?
As for "Continue where I left off", since you need to close each window one at a time in order to restart Opera I lose all tabs except for the last window I closed.
In Opera Classic this was called Sessions. Try saving your window layout as a session.

You just need to press Ctrl+Shift+X (or Opera -> Exit).
This will close all windows and keep your session as is.

Wow thanks, christoph142, this is useful!!

Same problem here. It happened out of sudden during my work. I closed last opened Opera window and quickly started opera again. The new window appeared but was completely black. It might be just a bug, so I closed the window (alt-F4), waited a while, started Opera again... It was still black, logged out and on to the workstation.. Restarted.. Reinstalled Opera.. Did Windows Update.. Nothing helped

This might be caused by an AutoUpdate, so i downgraded to Opera 19.0.1326.56. Everything worked fine. Upgraded back to current version of Opera (23.0.1522.75) - black window. So I switched to current version of Opera Next (24.0.1558.34).

Everything works well for now...

Answer:

How to use Opera browser to manage OperaMail's CA certificates

Suppose you already have the DER (.crt, .cer, .der) certificate file to be imported and OperaMail installed.

1. Install Opera browser 12.17
2. Exit OperaMail and Opera browser
3. Rename your original %appdata%\Opera\Opera\opcacrt6.dat to opcacrt6.dat.old
4. Move %appdata%\Opera Mail\Opera Mail\opcacrt6.dat to %appdata%\Opera\Opera\
5. Start Opera browser 12.17 and use it normally to import certs (tools, preferences, advanced, security, manage certificates, authorities, import)
6. Exit Opera browser
7. Move the opcacrt6.dat back to OperaMail AppData folder
8. Restore the opcacrt6.dat.old back to opcacrt6.dat

I imported the "avast! Mail Scanner Root" cert this way and it resolved the issue. If you want to work on Intermediate or Approved certificates, you should use opicacrt6.dat and opcert6.dat respectively.

I hope OperaMail is just a quick standalone build of Opera M2 client and will use Windows cert store like the new Opera does today.