xpcshell: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/lib64/xulrunner-devel-17.0.10/sdk/bin/xpcshell - from package: xulrunner-devel
/usr/lib/xulrunner-devel-17.0.10/sdk/bin/xpcshell - from package: xulrunner-devel

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/lib/debug/usr/lib/icedove-devel/sdk/bin/xpcshell - from package: icedove-dbg
/usr/lib/icedove-devel/sdk/bin/xpcshell - from package: icedove-dev
/usr/lib/debug/usr/lib/iceweasel-devel-38/sdk/bin/xpcshell - from package: iceweasel-dbg
/usr/lib/iceweasel-devel-38/sdk/bin/xpcshell - from package: iceweasel-dev

You may install the required package with command apt-get install {package}

Error in other languages:
xpcshell: bevel nie gevind nie
xpcshell: comandă negăsită
xpcshell: comando não encontrado
xpcshell: commande introuvable
xpcshell: command not found
xpcshell: käsku ei ole
xpcshell: không tìm thấy lệnh
xpcshell: komanda nerasta
xpcshell: Kommando nicht gefunden.
xpcshell: kommandot finns inte
xpcshell: komut yok
xpcshell: nie znaleziono polecenia
xpcshell: níor aimsíodh an t-ordú
xpcshell: no se encontró la orden
xpcshell: no s'ha trobat l'ordre
xpcshell: opdracht niet gevonden
xpcshell: parancs nem található
xpcshell: perintah tidak ditemukan
xpcshell: príkaz nenájdený
xpcshell: příkaz nenalezen
xpcshell: команда не найдена
xpcshell: командата не е открита
xpcshell: 命令找不到
xpcshell: コマンドが見つかりません