Running: C5 5.5.2.1
I recently spent several hours and knocked on several forum doors trying to find the answer to a problem:

After following the instructions in C5 under "Documentation" for adding @font-face to your C5 site:
http://www.concrete5.org/documentation/how-tos/designers/how-to-add...
I still couldn't get the fonts to show. The theme developer and the documentation author both came up with suggestions, and I finally got everything working in Chrome, Safari, IE - but not Firefox.
Without going over every one of the variations of src url's used in header.php, or the css, it all came down to removing the typographical customization possibilities available in the theme.

So, in "typography.css", instead of:

.c5wrap h1 {
/* customize_h1-font */font: 48px 'FranklinGothicFSDemi';/* customize_h1-font */
/* customize_h1-color */color: #6b6a6a;/* customize_h1-color */
margin-bottom: 20px; }

I did this:
.c5wrap h1 {
font: 48px 'FranklinGothicFSDemi';
color: #6b6a6a;
margin-bottom: 20px;
}

Be sure, before you change anything in a theme you have licensed, that you request permission to do so from the developer of that theme. This doesn't take a lot of time, is polite, and actually is legally required.
Also, before changing things in a custom theme, be sure to "fork" it- create a copy in the correct folder - also explained in detail under C5's documentation:
http://www.concrete5.org/documentation/how-tos/designers/modify-a-p...

I would like to add a big "thank you" to the developer of the theme and "growthcurve", who wrote the documentation for @font-face. They both responded very quickly to my calls for help.
I also welcome any additions or corrections to this post...