In Firefox 28.0 and Internet Explorer 11 (on Windows 7), .btn height is larger by 1px when @font-size-base is 16px.

see:

I tested it with https://gist.github.com/egy186/9751751.

Adding below will fix this issue, but this may not be the best way.

.input-group-btn > .btn {
  height: @input-height-base;
}