First, I hope you didn't install my add-on which does that: https://addons.mozilla.org/firefox/addon/move-active-tab-to-last/

Second, is it random for you, too, or consistently happens every time?

Third, do you see any evidence that Firefox 137's new tab group feature could be causing this (by adding the tab you activated into a group)? I have attached a screenshot of what a group looks like:

Firefox 137's new tab group

i dont use it

I have Sorts tabs advanced addon but i have been using it many years

and happens every day . i am going to try to check when. but it is not like if i press a window or if i click here

i am going to count the tabs next time