My question is - do you have the same?
I just tested it in OXCE 7.0 (https://openxcom.org/forum/index.php/board,22.0.html) and I'm also not seeing the two handed weapon arm firing animation where the soldier raises one arm.
(https://openxcom.org/forum/index.php?action=dlattach;topic=9744.0;attach=54286)
... however, when I look at the TFTD sprite sheets I don't actually see that 8 frame arm animation set.
I've attached two unit sprite sheets to this post and highlighted the 8 frames that UFO: Enemy Unknown / X-COM: UFO Defense uses for the two handed firing animation, unless I'm mistaken TFTD just doesn't have the animation.
(https://openxcom.org/forum/index.php?action=dlattach;topic=9744.0;attach=54282)(https://openxcom.org/forum/index.php?action=dlattach;topic=9744.0;attach=54284)