I've done some searching on this forum, and I know this has been mentioned many times before, but in all the threads I found they all devolved into arguments because different people had different ideas about what those features meant.

So, I wanted to put forward multiple requests in this thread, clearly defining each.

1) Browser Profiles

This is the most straightforward of the feature requests, having been implemented in most browsers. It allows for running additional instances of the browser, each with its own storage/extensions/cookies, etc. Great for keeping personal accounts separate from work ones.

• Chrome: https://support.google.com/chrome/answer/2364824
• Edge: https://support.microsoft.com/en-us/topic/sign-in-and-create-multiple-profiles-in-microsoft-edge-df94e622-2061-49ae-ad1d-6f0e43ce6435
• Firefox: https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

2) Multi-account containers or Cookie containers
This feature is far more complicated and niche than profiles, but is extremely powerful.
Cookie containers are used to either isolate or share cookies between tabs. This allows you to have a work persona and a home persona all in the same browser window.
This is what the people coming from Firefox want. You can have a number of different cookie containers and open a new tab or re-open an existing tab in a different container. These containers will have different cookie/local storage/etc data.
One of the profiles is usually marked as the default profile, for opening new tabs without any context, or for extensions.
To be clear, things like extensions

• Firefox: https://support.mozilla.org/en-US/kb/containers
• Wavebox: https://wavebox.io/blog/what-are-cookie-containers/ (Wavebox is a pay-for browser in which cookie containers are pretty much the base feature - though it's grown a lot beyond that single feature)
• Ghost Browser: https://support.ghostbrowser.com/article/320-identities (Ghost Browser is another chromium browser based around this feature)

3) Space-based containers

This one is far closer to Cookie Containers than it is to Browser Profiles. In fact, I'd argue that it's exactly the same as Cookie Containers, except that you assign the Cookie Containers to Spaces instead of individual tabs.
In this way, you could have a space with a bunch of websites logged into personal accounts, then a separate space where some of the same websites are logged into distinct work accounts.
I would suggest being able to assign Cookie Containers/Profiles to multiple spaces, if one wishes.

• Arc Browser does it this way: https://resources.arc.net/en/articles/6649632-profiles-separate-work-personal-browsing

All three of these features help people to use multiple accounts of the same website in different ways. Also, these three features would all be optional. Even if all three of these were implemented in one form or another, people who don't care for those features can stick with the default profile and the default cookie container and continue to use their browser exactly the same way they do today.
But for those of us coming from other browsers who rely on one or more of these features, it would be a real boon.

I sincerely hope that the effort I put into the details here shows how passionate I am about wanting these features, and the disambiguation contributes to a healthy discussion.
(I also hope this doesn't just get marked as a dupe and closed immediately)

My opinion is that Cookie Containers can be much more helpful than Browser Profiles, as you can use them in the same window context. I do also think that Space-Based containers are easier to manage though (sometimes having a window full of the same tab in multiple containers can be confusing, as niche a use-case as that is).
However Browser Profiles also have their place, with their own separate set of browser settings and extensions creating a unique work environment for different contexts.

I hope that any or all of these features are properly considered by the dev team. There are a great many of us out there who love Opera (especially Opera One - great work, guys!) but are still stuck using (or even paying for!) other browsers because a critical feature is missing.