DIV sizing screwed up in x86 www-client/firefox-bin-38.4.0?

[tld]

I'm a developer for a living and a lot of it is web development. Wow...have I ever been living a nightmare today...

I just wasted the better part of an entire day on this one. On the system I'm on now (an older x86 machine) I use firefox-bin, currently running www-client/firefox-bin-38.4.0. I had two situations today where divs were sizing incorrectly and almost lost my mind trying to figure it out, only to discover that I couldn't reproduce the issues on any other browsers, including other versions of firefox, even including firefox 38.4 on a CentOS VM (x86_64). It's clearly an issue specific to this bin version. The symptoms, though it could be almost impossible to pin down specifically, seem to involve some divs simply rendering larger than they should be.

Horrifying. Has anyone else run into this?

Tom