Antony wrote:Laurent quit the Firefox, and he can't restart it at all...

Of course, I checked the system process (Activity Monitor), Firefox was no longer running (no Firefox was running).
It happens on Windows too with both Ffx and SM, but is a rarity on either platform. What would appear
to be happening, is a small fragment of the program is stuck in system memory, some random dll file
or the Mac equivalent of dlls, so when you attempt to reopen the browser from the desktop icon on
the PC (or from the Dock on the Mac) that random fragment is preventing the relaunch.
Sometimes, you have to wait a few moments for that random fragment to eventually work itself loose,
before you click to restart your browser ... or just to close everything down and restart the computer
UserAgent: Mozilla/5.0 (Windows NT 5.1; rv:2.0.1) Gecko/20110608 Firefox/4.0.1 SeaMonkey/2.1
SeaMonkey = Swiss Army Knife: It's versatile, reliable, and contains useful tools.
Windows Internet Explorer = Old Swiss Cheese: Full of holes, and it stinks!