It means that the css changes are not taking effect.
1) create a custom css file.
2) save the css file within the site collection style library:
http://mysite:20034/style library
you can add a folder within the library or have it directly there!
3) open your masterpage that the site is using by downloading a copy.
4) add the custom css to the downloaded masterpage:
You must register the CSS file with a master page before the CSS file
can be used by the theming engine. This directs the master page to the
CSS file when you apply a theme to a site. To register a CSS file, you
add a element to the element of
the master page. The following shows the format of the
element.
<head>
…
<SharePoint:CssRegistration Name="<%$SPUrl:~SiteCollection/Style Library/~language/Themable/MyCustomFile.css%>" runat="server" />
</head>
http://msdn.microsoft.com/en-us/library/office/dn266906(v=office.15).aspx
WARNING** dont just download the default mastepage and overwrite it! if you are then rename the file to somthing else and set that new masterpage as the site collection masterpage!
ok, so now you have your custom css file and saved within the style library. You have a masterpage (custom) saved within the masterpage gallery that contains the new CssRegistration to you css file in the style library.
5) You now need to approve the masterpage and set it as major version (this is within /_catalogs/masterpage/Forms/AllItems.aspx where you add the masterpage and set it to major version)
......
now if you want to make some modifications to the css file just update it in the style library... to know what elements to ammend use F12 developer or firebug to inspect the elements on the page to see its id or class :)
EDIT
we know the css works as it does in ie but in firefox we now need to know what part is not working? im presuming all? if its all than you need to check firebug to see if the css is loaded into firefox browser? if it is than css is not applied and you need to add !important.
use this only if css file is added into firefox browser (check in firebug!), under css you should see Styles/fixedwidth.css, if it is then do:
#s4-workspace
{
background-color: silver !important;
}
#contentBox{
min-width: auto !important;
}
#s4-bodyContainer
{
width: 960px !important;
width: 60rem !important;
margin:auto !important;
background-color: white !important;
}
.ms-dialog #s4-workspace
{
background-color: white !important;
}
.ms-dialog #s4-bodyContainer
{
margin: 0px !important;
width: auto !important;
}
.ms-dlgContent #s4-bodyContainer
{
width: auto !important;
}
.clearfix
{
clear: both !important;
}