SourceSansPro-Bold font bolder in firefox

In Firefox:

.enum{
font-family: "SourceSansPro-Bold","Source Sans Pro";
font-weight: 700;
}

If I am doing font-weight: 400 than it is fine

.enum{
font-family: "SourceSansPro-Bold","Source Sans Pro";
font-weight: 400;
}

But when if user has the google fonts CDN blocked than it looks vary no bold

.enum{
font-family: "Source Sans Pro";
font-weight: 400;
}

Copyright Notice：Content Author:「S. Shukla」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/39269808/sourcesanspro-bold-font-bolder-in-firefox