There used to be a Firefox extension called FLST (Focus Last Selected Tab) that solved this problem, but that was years ago and I've yet to find a suitable replacement, ever since Mozilla rolled out web extensions and severely limited how much they could modify the browser.

I miss the early days of Firefox. It was a lot more fun.

BTW: Use the new Tab Groups feature if you haven't already. That way you can still have 100 tabs open while still keeping your browser neat and organized.