I was excited when Firefox 4 was released. I have used three of their released candidates and found lots of bugs. However, they did a great job of fixing all the bugs before the release. But one thing disappointed me a lot: improper display of বাংলা script. I frequently visit sites like প্রথম আলো, সচলায়তন etc. and write বাংলা in Gmail and Facebook. বাংলা script appeared in distorted form everywhere. The problem was partially solved when I downloaded the SolaimanLipi font from here. বাংলা in প্রথম আলো immediately began to be displayed correctly. However, problems with the rest of the sites were unresolved. Then I solved the problem by changing the default font to SolaimanLipi by going to Options -> Content -> Fonts & Colors -> Advanced. I selected "Bengali" in the "Fonts for" drop-down box. I selected SolaimanLipi for both Serif and Sans-serif. I hope this might solve problems faced by others too.
