Portable or not portable the interesting point is searching structurally inside all types of files from Thunderbird. That, of course, can be a portable program too.
But I only have in portable way. That is for me :
Install TB in a data disk (over my disk y: data disk, not in the system). Why ? Because in my old windows xp I share this installation between the differents installations under the same windows xp pro ....
I don't need to install in a usb drive, by example.
And I think too is an advantage because i have configuration and data in a data disk continually backing up to my security copies disk.
Is my system collapse I have the emails in other places. And I don't have to worry about so much installation in the system.
I try to install all i can in portable way. Even if the program have dependencies like net framework, java, etc.
Sometimes in "this portable way" i had to reinstall the program in every system because write fragments in the registry or even in some folders in the system.
Others cases for me had been when the installer only let me install on the system, but really the program is almost portable. So I copy the installation folder to my data disk and execute from there. Many times give me good results and no additional problems.
Usually some absolutely not portable programs are fat software.