Cannot change "Invalid" upstream bug tracker link

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The upstream Mozilla Firefox bug for bug #107247 was recently marked as a duplicate. After comment #66 in the Launchpad bug, the Bug Watch Updater changed the status to Invalid (sic) apparently as a result of this.

When I clicked the Mozilla Firefox task to replace the upstream bug link, I couldn't. The radio button for URL: was clickable, but I could not paste the correct link into the associated text field.

I eventually worked around this by clicking the radio button for "None, the status of this bug is updated manually" [0], then once the upstream tracker was editable again, clicking the URL: radio button and pasting in the correct link for the master upstream bug. This is somewhat documented in comment #67 of the Launchpad bug.

If the Bug Watch Updater were less eager to mark upstream bugs as "Invalid" (bug #56644) this would be a minor annoyance, but the way it stands, I think this needs to be fixed. If nothing else, it's confusing and hard to use. (I very nearly filed a separate bug requesting for an admin to change the upstream bug tracker link, before I figured out how to work around the problem.)

[0] Actually, the first one out of two identical ones; bug #516799. I don't really like how in that bug, a problematic experience is reduced to "oh, right, there shouldn't be two of those; we'll fix that at some point".