I am planning to use the Helios theme by HTML5up for a school project: http://html5up.net/helios/, but I noticed that Firefox (not Chrome, only Firefox) shows a cross inside of a box between navigation bar links, as shown here:
How can I change that? I must add I have Firefox Nightly and that I have noticed similar problems on other sites.
That symbol is what results when Firefox fails a CORS request for a webfont. Can you check your network panel and confirm that all of the required webfonts are loading properly?
I'm not able to reproduce this in Firefox Nighly on Linux with v29.0a1 (2014-01-08).
Can you confirm that you don't have any extensions that may be conflicting with your pages?