Selenium WebDriver does not just check for opacity != 0, visibility = true, height > 0 and display != none on the current element in question, but it also searches up the DOM's ancestor chain to ensure that there are no parent elements that also match these checkers. (UPDATE After looking at the JSON wire code that all the bindings refer back to, SWD also requires overflow != hidden, as well as a few other cases.)
I would do two things before restructuring the code as @Brian suggests.
Ensure that the "div.modal_footer" element does not have any reason for SWD to consider it to not be visible.
Inject some Javascript to highlight the element in question in your browser so you know absolutely you have selected the right element. You can use this gist as a starting point. If the button is highlighted in a yellow border, then you know you have the right element selected. If not, it means that the element selected is located elsewhere in the DOM. If this is the case, you probably don't have unique IDs as you would expect, which makes manipulation of the DOM very confusing.
If I had to guess, I would say that number two is what you are running into. This has happened to me as well, where a Dev reused an element ID, causing contention in which element you're supposed to find.