A lot of complain about Bangla font viewing can be found on various blogs and forums. After many days I've found a solution to Bangla font viewing problem on mozilla firefox. This solution is the most effective and tested on several computers, having different OS. So now let's see the solution.
First of all, you'll need to have the font, 'SolaimanLipi'. Download the font from here. Secondly, you'll have to download Avro Keyboard and install it on your computer. After installation, go to the folder where you've installed Avro keyboard. It's probably in C:\Program files\Avro Keyboard\. Here you'll find a file named Font fixer. Click on it. When it will open, select 'SolaimanLipi' from the drop-down menu of 'Choose your font'. Now click 'Fix it. Then you'll be asked to restart PC to take effect. Restart your PC. That's all about font configuration on system.
Now let's configure Firefox. Here first of all, click 'Tools' from the menu bar and then click 'Options'. If you don't see menu bar, click 'Firefox' on the top right side of your firefox window. Here you'll see a bunch of functions in two columns. On the 2nd column you'll see 'options'. Click it. Then click 'Content' tab. Here On 'Fonts & Colors' tab, select 'Times New Roman' as default font. Now click 'Advanced'. Here make changes like the setting given below:
Fonts for : Western
Proportional : Serif
Serif : Times New Roman
Sans-serif : SolaimanLipi
Monospace : Tahoma
Place a tick mark on the check box of 'Allow pages to choose their own fonts, instead of my selection above'. Leave it without any change if you already see the tick mark on the check box of this option.
Now select 'Unicode (UTF-8)' as default Character Encoding on 'Character Encoding' tab. Now click OK. and close the options again pressing OK.
That's all. Now restart your browser and then you'll find that all your Bangla font viewing related problems have been solved.