What is strange here, that Thunderbird some times still executes browser directly, as in this case,
clicking the link inside notification email from github.com:
```
$ sudo sysdig "proc.name=thunderbird and evt.type=execve"
150186 18:52:01.190105156 0 thunderbird (11430) > execve
filename=/usr/bin/firefox
```
While if I click link inside my local mail send from myself to vincas@localhost:
```
2346630 18:52:29.734742813 3 thunderbird (11552) > execve
filename=/usr/lib/x86_64-linux-gnu/glib-2.0/gio-launch-desktop
2346632 18:52:29.734789809 3 thunderbird (11552) < execve res=-13(EACCES)
exe=/usr/lib/x86_64-linux-gnu/glib-2.0/gio-launch-desktop
args=/usr/lib/firefox/firefox.http://duckduckgo.com/. tid=11552(thunderbird) pid=11552(thunderbird)
ptid=11551(thunderbird) cwd= fdlimit=1024 pgft_maj=0 pgft_min=80 vm_size=2025636 vm_rss=254608
vm_swap=0 comm=thunderbird
cgroups=cpuset=/.cpu=/.cpuacct=/.io=/.memory=/user.slice/user-1000.slice/session-4.sc...
env=FORCE_MIMEAPPS_MIGRATE=0.GS_LIB=/home/vincas/.fonts.KDE_FULL_SESSION=true.LAN... tty=0
pgid=2283(kdeinit5)
```
