
Typically, I download .xpi, move .xpi into my XPI directory, then install.
Today, download, install, then went to move.
The move "errored".
Then I look & I see the .xpi is in my XPI directory.
But it is also in my download directory.
(So it did the copy part [different drives], but not the "move" [delete] part.)
Checked, OpenedFilesView, but that did not see it.
Process Hacker did.
I was able to rename the file (first from noscript-5.1.2rc3.xpi to noscript-5.1.2rc3.xpi.xxx, then to noscript-5.1.2rc3.xpi.xxx,y), but not delete it - until I had first closed SeaMonkey.
Likewise, until I restarted SeaMonkey, I was unable to upload my screenshots as the upload button ("Choose images") was ineffectual.

I didn't check to see if {73a6fe31-595d-460b-a920-fcc0f8843232}.xpi was (also) listed in there, but suspecting it had to be? Also seems there is not a straight forward set of circumstances where this manifests; as in, new Profile install & check probably won't cause the issue.
