Why do some Mozilla extensions require me to uninstall manually when overinstalling/upgrade them?

Hi,

I noticed some extensions for Mozilla v1.7.x requires me to manually 
remove the old installed extensions before overinstalling/upgrading due 
to "chrome" errors and others. Why does Firefox doesn't require this? I 
have seen this in all platforms (Windows 2000 and XP, MacOS X 10.2.8, 
and Linux/Debian).

Thank you in advance. :)
-- 
"..., you ready for a little dumpster diving?" "Um... okay." "You know I 
don't mind getting my hands dirty." "I mean, maggots, wet trash, I am 
the first one in." "Okay, so what are you waiting for?" "Ants." 
(Chuckles) "Ants?" "Yes, I have got a problem with ants." "They are 
sneaky, and they are mobile, and when they get on you, even if you get 
them off..." "Okay, Calleigh, chill." --CSI: Miami (Wannabe episode; #218)
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