I'm seeing a strange scenario where, according to the Firefox dev console, the matched color for a css element is different from the visible output. If it matters, I'm using Bootstrap 3 as the source of styling. Here is the (very short) HTML file that I'm using for the example.
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css" rel="stylesheet">
<button class="form-control btn btn-success">Hello world</button>
Interesting observation! It would appear that the Firefox developer tools use the line number of the declaration block to determine selector importance, when the specificity of two selectors is the same.
This can be an issue when the CSS file is minified though, and all declaration blocks end up on the same line.
It becomes more apparent if you use the non-minified resource:
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.css" rel="stylesheet">
Hope this sheds some light on the issue. Maybe it's worth a bug report?
EDIT: A bit more testing indicates that the Firefox tools places precedence on the left-most block when faced with multiple same-specificity selectors on a single line. (An example.)