Right click a tab and select 're-open last closed tab' multiple times to restore as many 1 by 1 as you can (or press CTRL+SHIFT+T). Unless leocg is saying this won't work after Opera has been closed. Worth a try if not.

Tip: Occasionally right click tabs and select 'save all tabs to speed dial folder,' to have backup copies.

I also do a lot of tabs as a habit of mine too.

P.S. You can check your history to re-find a lot of the lost tabs also, depending on their age especially.

You'll have issues with FireFox from time to time just like any other application, Opera included; but, maybe that's the only solution for you at this time, aside from re-logging into everything manually (once).

I also prefer a machine-side copy of everything (so I do Opera sync + an encrypted PC file).

If not Opera's or another party's ad-blocker (i.e. AB+), it could be an anti-virus (device app like Bitdfender, or browser extension like Avira). Sometimes it can be a router/internet issue, like I've had a laggy connection do that, and I've had a phone hotspot wifi that wasn't allowing FTP before. Random thoughts. Good luck.

Welcome to Opera, I love it.

So, are you trying to import bookmarks and passwords FROM Chrome and Firefox TO Opera?

When you first install Opera I think it gives you a chance to then, and that may even work better than the settings > import approach. Not sure, however.

I wonder if Chrome and FireFox also utilize the same file structure as Opera. Inter-compatibility may not be an option, equally the fault of each product, but hopefully for simplicity it is.

You say you got your Chrome bookmarks successfully, but not the passwords from Chrome, and neither from Firefox?

Is there any type of error or message you get when trying the ones that do not work?

Once you get all your bookmarks and passwords into Opera, I recommend signing in and selecting synchronize to add a pass phrase to sync everything. So every new machine or OS you simply install Opera, log on and sync, and bam, all bookmarks and passwords from the server/cloud, using an encryption key only you have (not even Opera knows).

As for now, there's an Opera blog that may help: https://blogs.opera.com/news/2014/12/how-to-import-bookmarks-from-another-browser/

For me that was a blocked advertisement that looked like that ^ when that happened to me. Except for a time it was an Internet connection issue. But if that part of the site is supposed to load, then perhaps it's being mis-perceived as an ad by Opera's ad-blocker. Or it may not be Opera at all, an anti-virus could've felt it was dangerous.

Try disabling ad-blocker on Opera (a great feature, don't get me wrong), and then see if it works.

This is indeed the Linux, not Windows, forum location.

I've used Opera on Linux for a few years now, and I recommend you solve this problem by signing into Opera on both machines/browsers (you seem to already have an account).

Then set synchronize to include everything from bookmarks to passwords (you get to set your own passphrase by which they are encrypted, so even Opera doesn't have the key, it's quite secure).

Every new machine / OS install I simply download Opera and log on and and sync and boom, all my bookmarks and passwords are there.

That said, it sounds like the 'export' and 'import' options didn't automatically work for you. And you already tried one fix (the flag approach), without any luck. I wonder if the file is compatible between both Operas you have. The sync approach I noted works every time, on that note.

Good luck.