I've been using Opera since version 4, I think. I very much like it and have chosen it over Chrome, IE, Edge, and Firefox. I especially liked tabs, re-open with previous tabs, and mouse gestures, all features that I saw first on Opera and then were adopted by other browsers. I was disappointed when mouse gestures were removed from the core product, but tolerated the change.
I recently learned from a colleague about the Containers feature in Firefox, As much as I like Opera, that feature has me seriously considering giving Firefox a go for the first time since the '90s.
I am an IT admin and all day long, every work day I am constantly logging in and out to different clients' resources. It isn't unusual for me to have an Opera session open, another Opera Private session open, and sometimes another browser + their private session windows open as well.
The Firefox feature is described here: https://addons.mozilla.org/en-US/firefox/addon/multi-account-containers/.
I'd LOVE to see something like that for Opera.