I am facing the same problem. I solved the problem for http://www.prothom-alo.com by downloading their font. But for all others (e.g. Facebook, Gmail email, email compose text box), the problem is there.

well i was having same problem. but this can be solved easily.

step 1: download Avro from http://www.omicronlab.com/avro-keyboard.html then you have the Unicode bangla font you need(if you have this before hand then go to step 2)

step 2: click Firefox button on the top left corner then goto option > option > content. then in the Font&Color tab select your font into Siyam Rupali or any other Unicode bangla font. problem solved for most site like facebook,Gmail and Bangla Blogs.

but for site which don't use Unicode font , font used by them has to installed manually.

Download Bengali fons from here : http://www.nongnu.org/freebangfont/

Install them. This will resolve.

Visit this page to check whether they are working : http://www.bhalobasa.in/articles/119540/Autograph-Lyrics-in-Bengali-Script-

While I was having the problem, I was already using Ovro and had the font "Siyam Rupali" selected. I changed the font to "SolaimanLipi" provided by Prothom Alo and my problems with Facebook, Gmail and Bangla blogs are gone now.

all you need to do is just change the font of "bengali" language of your firefox...its 'siyam rupali' in default..and firefox 4 and upper versions have some prob with this font...

Go to firefox option..then proceed to content>font and colour>advance...then select "bengali" in font for section..then you can see the 2 default fonts for bengali..just change these 2 fonts into "Adorsholipi"...nothing more to change...select ok and quit..now your prob is solved...:p:p

check the picture below to have a clear idea...:P