I have installed portableapps.com Firefox and Thunderbird on the Boxcryptor drive (windows 7 x64, boxcrptor 188.8.131.52).
Comodo Internet Security (comodo.com) is installed.
When the Comodo antivirus is enabled, Firefox portable and Thunderbird portable crash just after startup.
When the Comodo antivirus is disable, the apps work fine.
When I copy the portable apps folder to a directory on my HD (outside of Boxcryptor), the apps work fine with the Comodo antivirus enabled.
So there seems to be a conflict between boxcryptor and comodo antivirus.
These apps are all free to download and use, you can test this on your system to reproduce the issue:
1. install http://download.comodo.com/cis/download/installs/1000/standalone/cispremium_installer.exe
2. install in boxcryptor http://downloads.sourceforge.net/portableapps/PortableApps.com_Platform_Setup_10.0.1.exe
3. install in boxcryptor http://portableapps.com/apps/internet/firefox_portable
4. install in boxcryptor