After the latest edits in CSS/HTML of (Meta) Stack Overflow the
<input type="file"...> is too wide. See the screenshot:
I use Firefox 18.0.1 on Ubuntu 12.04.
As Shog9 commented, this is basically a bug in Firefox - it will not respect width styles on file inputs.
I have added a size attribute on the file input so it shouldn't go over the dialog width, but YMMV (the Windows and Linux versions of FF do different things here). There is also the issue of what fonts and font sizes you are using etc...
Tested on Linux Mint, FF 19 and will be there on the next build.