The file is fine.  Firefox doesn't like the .exe file.

I'm assuming you're using Windows.  If so there are two downloads on the Boolberry downloads page.  One is "Full Installer" and the other is "Full Package".  The "Full Installer" is an .exe file and that is what's causing Firefox to give you a warning.  The "Full Package" is a zip file.  I personally don't like using installers if I don't have to so I use the "Full Package" and unzip to a directory of my choosing and run the qt-boolb.exe from there.  Maybe use the "Full Package" .zip download and then do a mandatory check of the .exe at virustotal.com before launching.  Then just wait for it to download the blockchain which will take some time.

You can also use the command line simplewallet which is included in the "Full Package" .zip download.  http://boolberry.com/howto.html#wallet