After I installed 360 Standard Explore, it removed my default internet browser from FireFox to Internet Explorer in start menu. My operation system is still old Windows XP on this computer.
I first checked in FireFox Tools->Options->Advanced->General, it’s still shows:
Always check to see if Firefox is the default browser on start up — Firefox is currently your default browser.
So the issue is not here. So I ran regedit from run command, and I checked
HKEY_CURRENT_USER->Software->Clients->StartMenuInternet does set (default = FIREFOX.EXE)
Then I searched
HKEY_LOCAL_MACHINE->SOFTWARE->Clients->StartMenuInternet, and found FIREFOX.EXE key are setting correctly. But HKEY_LOCAL_MACHINE->SOFTWARE->Clients->StartMenuInternet default = IEXPLORE.EXE, this is why system did not run FIREFOX.EXE settings. So clear default value (now shows Default = value note set), then system will run Firefox when you click Internet from start menu.
The issue fixed.