It is the current Firefox with the current Zotero on a Kubuntu 14.04.4 LTS on a 32-bit machine.
Just a hint: I think the Zotero-AddOn use the firefox default font and its size here (in my case Dejavu Sans 16pt). But it use a (in pixel) fixed size of the drop-down element without taking care of the used font.
We test on OS X, Windows, and stock Ubuntu. If you can provide steps to reproduce the issue with one of those, we'll take a look. Otherwise, you'll have to debug this yourself. You're welcome to submit a pull request on GitHub with a fix.
Just a hint: I think the Zotero-AddOn use the firefox default font and its size here (in my case Dejavu Sans 16pt). But it use a (in pixel) fixed size of the drop-down element without taking care of the used font.