Just tested and confirmed it's the same in Firefox 78(esr): the [x]'s disappear after (on my screen size) 13 tabs. So it's been this way for quite a while.

Why use the 'x' when you can just middle click to close?

I also like the new look. I've had a hard time quickly identifying the active tab so this way seems best to me.

I use a MacBook with a touchpad so there is no middle click (at least not easy to do)

I thought a mac might be involved somehow but didn't consider touchpads. I guess even PCs have annoying touchpads with no real buttons these days. I really can't stand touchpads in general and avoid them. Unfortunately even Lenovo seems to rarely have trackpoints these days.

Three-finger-click works fine for me using an Apple wireless trackpad on Linux, doesn't that work for you?

I personally find three-finger clicking on a tab to be more difficult than one-finger clicking on the x.

I think I have that disabled as I don't find it very easy to do.