using two users in a script and the synchronization issues

[esolve]

the following is a script snippet in my script
I run this script using root, coz tcpdump need root priviledge
but I need to use "sudo -u regular_user" to run firefox

while read line
do
  sudo -u regular_user App/Firefox/firefox --profile Data/profile &
  sleep 2
  cd ${capture_dir}
  tcpdump -i eth0 -w capture${i}.pcap &
  cd ${firefox_dir}
  sudo -u regular_user App/Firefox/firefox --profile Data/profile -new-tab case.html
  sleep 200
  pid_firefox=`ps -ef|grep "firefox"|grep -v "grep"|awk '{print $2}'`
  kill -9 $pid_firefox
  sleep 15
  pid_tcpdump=`ps -ef|grep "tcpdump"|grep -v "grep"|awk '{print $2}'`
  kill -9 $pid_tcpdump
  sleep 3
  i=$((i+1))
done < $url_list

I noticed that the process execution sequence is not as expected
sometimes firefox processes are not all killed and then the next loop has already started, can anyone tell me are there any process synchronization issues in my script?
thanks!

[Cancerous]

You can use && or || to tell a command to run once the previous command was successful or failed.

By using a single ampersand, you're telling bash to run that process in the background and begin the next immediately.

Ampersands separate asynchronous commands. An ampersand does the same thing as a semicolon or newline in that it separates commands, but it causes Bash to execute the first command asynchronously, which means that Bash will run it in the background and run the next command immediately after, before waiting for the former to end.

Source: BashSheet - Greg's Wiki