I need a solution for this, as well. I work primarily on a laptop and use the trackpad instead of a mouse - I often accidentally close important tabs after simply trying to navigate to them. This has been going on for years and I can't find a way to fix it. I'm running the latest Firefox on Ubuntu 22.04.

Trackpad users run into this problem because the middle-click is less distinct on a trackpad than it is on a mouse, so a feature such as "close tab" that can't be turned off is not just bad UX but also presents accessibility issues. The ability to disable middle-click close would benefit the community greatly.