XPI won't install on its own

[email protected] (Scorpius)
Newsgroups gmane.comp.mozilla.devel.xpinstall
Organization http://groups.google.com
Message-ID <[email protected]> 
On some occasions clicking a .xpi file on Firefox's extensions page
will open a dialog box asking me how I would like to handle the file
(save to disk, open with...) -- that is, it won't start the install
process. Sometimes it helps to restart the browser, sometimes this
won't help at all.
Is there any workaround for this issue? Or is there a way to install
.xpi file "directly" if it's saved to disk?
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