Just follow this tip
"If you get a dialog box to confirm installation but the extension
isn't listed after choosing to install (or if you update an extension
but it's still the previous version after restarting), exit Firefox.
Go to the Firefox<b> profile folder</b> and look inside the
"extensions" folder. If you find a <b>staged-xpis</b> subfolder,
delete it. Reopen Firefox and try again [23]. "
You can find out the profile folder in the direction:
<i>C:\Users\<Windows login/user name>\AppData\Roaming\Mozilla\Firefox
\Profiles\<profile folder></i>
An old bug from firefox 1.5
On Sep 3, 2:45 pm, david <
michaelg...@gmail.com> wrote:
> Hi all,
>
> When installing the latest firebug i got the message:
>
> Firefox could not install the file athttp://
getfirebug.com/releases/firebug/1.5X/firebug-1.5X.0a23.xpi