i cropped down an image and selected it to be aligned center. the caption is also aligned center but when I hit preview they are not both aligned together in the center. not sure why but this happens kind of often.
It may be worth double checking that the Firefox browser version is current. Going to https://browsehappy.com/ will tell you that. Also, disable any extensions in that browser to see if it makes a difference at all.
Noting this was happening in my current FireFox on mac. It could be related to a bug in Firefox’s “table-caption” support. Can you try adding this to your CSS to see if it helps? I wasn’t able to test it fully, but preliminary tests look like it may do the trick.