1. Open Firefox and go to Preferences as the image.
2. Click on Content and then Advanced.
3. Make these changes in the box appears:
a. Select drop down of Fonts for to Bengali.
b. Serif to Ekushey Lohit.
c. Sans-Serif to Ekushey Lohit.
d. Monospace to Ekushey Lohit.
e. Default Character Encoding to Unicode (UTF-8).
You are done now.
Prothom-Alo on my MAC os X Screen
Kalerkantho on my MAC os X Screen
All Photo Here (Click to View full size.)