On Linux with Nightly 95.0a1 (2021-10-19) (64-bit), on either X or Wayland, in a window with at least two tabs open, clicking and dragging any tab results in that tab being moved to the leftmost position in the window, regardless of the drag destination. Steps to reproduce:

1. Launch Firefox.
2. Open two new tabs. (e.g. Press Ctrl+T twice)
3. Browse to Wikipedia (or any website - so it can be distinguished from the others) in the rightmost tab.
4. Click and attempt to drag the rightmost tab to move it between the other two tabs (into the middle).
5. Observe that it moves to the leftmost position and can not be dragged to the middle or rightmost position.

Mozregression:

Last good revision: 1117eee12de7907d215343eafb1156d50e4b7ab5
First bad revision: f4a1cfc1bb0f7d72baa621c2a145358b74de8945
Pushlog: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=1117eee12de7907d215343eafb1156d50e4b7ab5&tochange=f4a1cfc1bb0f7d72baa621c2a145358b74de8945

Suggesting that the bug was regressed by Bug 1736377.