কিভাবে ফায়ার ফক্সে বাংলা ফন্ট দেখবেন (How to see bangla / bengali fonts in Firefox)

To fix it in your latest firefox versions (22.0 or later) simply go to
tools > options > content
there you will see the fonts & color menu in the mid section
click on advanced and you will go to advanced fonts field
select the following:
font for: Bengali size:16
proportional: Sans Serif
serif: Siyam Rupali
sans serif: Siyam Rupali
Monospace: Siyam Rupali size:14
DO NOT UNCHECK "Allow pages to select their own fonts, ..." checkbox
and then at the bottom
Set Fallback Character Encoding: Western (ISO-8859-15)
Click ok and ok and voila there you are. All of your problems has been fixed :)