Make sure that you allow pages to choose their own fonts.

• Tools > Options > Content : Fonts & Colors > Advanced: [X] "Allow pages to choose their own fonts, instead of my selections above"

Any errors in the Net tab in the Web Console (Firefox/Tools > Web Developer;Ctrl+Shift+K) about this font?

Is this a font downloaded via @font-face or a locally installed font?

it was a locally installed font and i get an error message about font in the console. here:

I still couldn't solve my problem. Can anybody help me out?

You will have to download and install the font(s) locally.

Also make sure to specify the local property to make Firefox use a local file instead of downloading the fonts, otherwise it only works if special HTTP access control headers are send.
You can also remove any @font-face rules to make Firefox use the locally installed fonts.

@font-face { font-family: '<font name>'; src: local('<font name>'); }

• https://developer.mozilla.org/en/CSS/@font-face
• https://developer.mozilla.org/En/HTTP_Access_Control

Did you complete your registration on this forum by going to the link in your confirmation mail (assuming that you received such a mail)?

I ask because this thread doesn't show on the forum question index page and in my own contributions and that makes it difficult to keep track of it.

It only shows in a search:

• https://support.mozilla.org/en-US/search?q=&asked_by=rangertr&w=2&a=1
• https://support.mozilla.org/en-US/search?q=font&asked_by=&answered_by=&w=2&a=1&sortby=1