Fram pointed this out at en.wp, this seems to affect FF.
Steps to reproduce:
- try to change the caption of the only picture at https://en.wikipedia.org/wiki/Giovanni_Berchet?veaction=edit (for example, add the letters "no" after "Giovanni");
- notice the "Apply Changes" button doesn't get enabled;
at this point you can try one of the following,
- remove the letters you just added by hitting Backspace: the B of the surname also gets deleted;
- instead of removing the letters, add something to the "alternative text" field. You can now click on "Apply Changes"; a preview will show you the alternative text was succesfully added, but no changes to the caption.