opera-beta_34.0.2036.16_amd64.deb is already working with Firejail (0.9.34-1) in Ubuntu 15.10 if it is invoked with the following --noprofile option :

$firejail --noprofile opera-beta

Firejail is a must especially if you have installed Flash-plugin in Opera.

Firejail – A Security Sandbox for Mozilla Firefox, Part 1

https://l3net.wordpress.com/2014/09/19/firejail-a-security-sandbox-for-mozilla-firefox/

"Chromium sandbox is similar to Firejail, it implements its own seccomp filter, but it leaves the filesystem wide open."

To give us understanding on the Chromium sandbox implementation, enter the following on the Chromium URL address bar on a plain Chromium running without Firejail.

file:///home/

Try to browse your home directory. Your home directory and the whole /home directory are accessible.

Now lets try running Chromium inside a Firejail via CLI or on the terminal :

$firejail --private chromium

Then access your home file directory by entering on the Chromium URL address bar

file:///home/

Here is another option, create a sandboxed folder in your home directory then run the following:

$firejail --private=/home/user/sandboxed chromium

Replace user with your username account, .

Access again your home file directory by entering on the Chromium URL address bar to see the difference.

file:///home/

The above example can also be easily done on Firefox using the File menu instead of entering on the URL address bar.

If you want to, make sure to disable seccomp sandboxing in firejail.