Masonry does not place images optimally in Firefox

Masonry works well in other browsers except Firefox. In Firefox, it distributes images incorrectly, leaving large columns of white space unfilled.

In Firefox 34.0.5:

Correct behavior in Chrome:

Here is the relevant Masonry code: https://github.com/codeforboston/bostongreenmap/blob/master/client/js/app.js#L354-L360

Here is the relevant CSS code:
https://github.com/codeforboston/bostongreenmap/blob/master/client/scss/_results.scss

Relevant Handlebars template:
https://github.com/codeforboston/bostongreenmap/blob/master/client/templates/resultItem.hbs

And Masonry docs: http://masonry.desandro.com/

We need Firefox to place the images as they are placed in Chrome and other browsers. Any thoughts on what is causing this?

Here is some possibly related discussion: https://github.com/desandro/masonry/issues/166#issuecomment-3479151
This issue concerns percentage widths, so it doesn't seem relevant.