After updating from "about firefox",there're always two version showing up in my control panel's program list.

Question

I think everytime firefox update itself, it doesn't clean the old version's register key so there're both two version showing up in the control panel's program list.
When you uninstall the old version, the new one disappears too.

Answer 1

Firefox can find plugins in several locations, but Firefox 21 changed the location of the "shared" plugin folder so older installers like the Microsoft Windows Media Plugin no longer drop the DLL file in the correct location.
There apparently are two ways to address this:
(1) Change a Firefox preference so that Firefox checks the old location. Here's how:
(i) In a new tab, type or paste '''about:config''' in the address bar and press Enter. Click the button promising to be careful.
(ii) In the filter box, type or paste '''plugins''' and pause while the list is filtered
(iii) Double-click '''plugins.load_appdir_plugins''' to switch its value from false to true.
This will take effect after you restart Firefox.
(2) Copy the plugins folder to the new location. Here's how on Win 7 64-bit:
Open a Windows Explorer window to:
C:\Program Files (x86)\Mozilla Firefox
Right-click and copy the '''Plugins''' folder
Double-click the '''browser''' folder to open it
Right-click and paste
Right-click the new copy of '''Plugins''' and rename it to '''plugins'''
After restarting Firefox, the plugins in that folder should now be available.
''Edit: I suggest just doing #1.''