Thank you for the detailed report. Certain webfonts are known to be incompatible with 10.4 and 10.5. Unfortunately, due to other limitations, it is not possible to detect them automatically. We maintain a blacklist of bad fonts, and the browser will select an alternative with the same character.
Using the steps in this earlier support ticket, are you able to find the font URL that causes the problem for you? I can make a guess from the Apple URL, but not the Adobe one. If you can provide both, I will add them to the internal blacklist for the next release of TenFourFox.
The Apple URLs will be blocked. Thank you for performing those steps.