Masonry does not place images optimally in Firefox

[allthesignals]

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: desandro/masonry#166 (comment)
This issue concerns percentage widths, so it doesn't seem relevant.