From Firefox for Linux, there is not only a 64bit-preview of Firefox 4, also the version 3 is in 64bit. The project is called Namorka [...]
There is no 64-bit-Firefox project called "Namorka". But "Namoroka" is the code name for Firefox version 3.6 as "Shiretoko" was the code name for FF 3.5 and as "Tumucumaque" is for the upcoming FF 4.0.
[...]Concerning flash, since five weeks there is a newer version available in 64 bit (10.2.), and until today I didn't remark any problem when using it [...]
Not detecting any problem does not mean there are no bugs. Thats the same as when you're coming back to your home after work, saying the door is still locked, therefore nobody was in. -- Wrong! Even when you unlock and open the door you can't be sure that there wasn't anybody in.
[...] What you forgot to mention, using a 64bit-browser means a high level of security, because at the very moment there is no suitable malsoftware for this architecture (under windows).
PC security is one of my main concerns. I already wrote that in some of my posts here.
A 64-bit os does not guarantee higher security because most 32-bit applications will also work on it; not just on Windows but also on other os's.
Increased security can be reached by driver or software signing, the step MS is doing with its 64-bit os's. But this does not just have pros but also a lot of cons: You can't install any driver/software that is not signed by the os vendor. If you release a driver or an application that runs on a only-signed-sw-allowed-os you have to pay expensive recurrent licence fees to the os vendor for letting sign your product.
Signing software on open source systems is useless because everyone knows how to sign, therefore everyone is able to sign: You can create your own SSL certificate thanks to the openSSL library, but that's no guarantee that it will not misused once by someone.