CSSPopup is a small plugin for firefox. It appends a new contextual menu opening a new window containing the CSS properties of the element that was clicked. For example , clicking on "Welcome to NCBI" at http://www.ncbi.nlm.nih.gov/ returns

h1 {
    font-size:32px;
    font-weight:bold;
    line-height:36px;
    margin-bottom:21.4333px;
    margin-top:21.4333px;
    padding-left:16px;
    -moz-column-gap:32px;
    }

div {
    }

div {
    }

div {
    }

div {
    margin-bottom:0px;
    margin-left:0px;
    margin-right:0px;
    margin-top:0px;
    }

body {
    margin-bottom:8px;
    margin-left:8px;
    margin-right:8px;
    margin-top:8px;
    }

html {
    background-attachment:scroll;
    background-color:transparent;
    background-image:none;
    background-position:0% 0%;
    background-repeat:repeat;
    border-collapse:separate;
    border-spacing:0px 0px;
    bottom:auto;
    caption-side:top;
    clear:none;
    clip:auto;
    color:rgb(0, 0, 0);
    content:none;
    counter-increment:none;
    counter-reset:none;
    cursor:auto;
    direction:ltr;
    display:block;
    empty-cells:show;
    float:none;
    font-family:serif;
    font-size:16px;
    font-size-adjust:none;
    font-style:normal;
    font-variant:normal;
    font-weight:400;
    height:auto;
    ime-mode:auto;
    left:auto;
    letter-spacing:normal;
    line-height:19px;
    list-style-image:none;
    list-style-position:outside;
    list-style-type:disc;
    margin-bottom:0px;
    margin-left:0px;
    margin-right:0px;
    margin-top:0px;
    marker-offset:auto;
    max-height:none;
    max-width:none;
    min-height:0px;
    min-width:0px;
    opacity:1;
    outline-color:rgb(0, 0, 0);
    outline-offset:0px;
    outline-style:none;
    outline-width:0px;
    overflow:visible;
    overflow-x:visible;
    overflow-y:visible;
    padding-bottom:0px;
    padding-left:0px;
    padding-right:0px;
    padding-top:0px;
    page-break-after:auto;
    page-break-before:auto;
    pointer-events:visiblepainted;
    position:static;
    quotes:"“" "”" "‘" "’";
    right:auto;
    table-layout:auto;
    text-align:start;
    text-decoration:none;
    text-indent:0px;
    text-rendering:auto;
    text-shadow:none;
    text-transform:none;
    top:auto;
    unicode-bidi:normal;
    vertical-align:baseline;
    visibility:visible;
    white-space:normal;
    width:auto;
    word-spacing:normal;
    word-wrap:normal;
    z-index:auto;
    -moz-appearance:none;
    -moz-background-clip:border;
    -moz-background-inline-policy:continuous;
    -moz-background-origin:padding;
    -moz-binding:none;
    -moz-border-bottom-colors:none;
    -moz-border-left-colors:none;
    -moz-border-right-colors:none;
    -moz-border-top-colors:none;
    -moz-border-image:none;
    -moz-border-radius-bottomleft:0px;
    -moz-border-radius-bottomright:0px;
    -moz-border-radius-topleft:0px;
    -moz-border-radius-topright:0px;
    -moz-box-align:stretch;
    -moz-box-direction:normal;
    -moz-box-flex:0;
    -moz-box-ordinal-group:1;
    -moz-box-orient:horizontal;
    -moz-box-pack:start;
    -moz-box-shadow:none;
    -moz-box-sizing:content-box;
    -moz-column-count:auto;
    -moz-column-gap:16px;
    -moz-column-width:auto;
    -moz-column-rule-width:0px;
    -moz-column-rule-style:none;
    -moz-column-rule-color:rgb(0, 0, 0);
    -moz-float-edge:content-box;
    -moz-force-broken-image-icon:0;
    -moz-image-region:auto;
    -moz-outline-color:rgb(0, 0, 0);
    -moz-outline-offset:0px;
    -moz-outline-radius-bottomleft:0px;
    -moz-outline-radius-bottomright:0px;
    -moz-outline-radius-topleft:0px;
    -moz-outline-radius-topright:0px;
    -moz-outline-style:none;
    -moz-outline-width:0px;
    -moz-stack-sizing:stretch-to-fit;
    -moz-transform:none;
    -moz-transform-origin:50% 50%;
    -moz-user-focus:none;
    -moz-user-input:auto;
    -moz-user-modify:read-only;
    -moz-user-select:auto;
    -moz-appearance:none;
    -moz-user-select:auto;
    }

Installation

Download CSSPopup from http://code.google.com/p/lindenb/downloads/list and open/install it with firefox.