[tbb-bugs] #28375 [Applications/Tor Browser]: improve handling of uninstalled protocol handler

[Tor Bug Tracker & Wiki]

#28375: improve handling of uninstalled protocol handler
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     Reporter:  mcs                       |      Owner:  tbb-team
         Type:  defect                    |     Status:  new
     Priority:  Medium                    |  Milestone:
    Component:  Applications/Tor Browser  |    Version:
     Severity:  Normal                    |   Keywords:  ff60-esr
Actual Points:                            |  Parent ID:
       Points:                            |   Reviewer:
      Sponsor:                            |
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 In https://bugzilla.mozilla.org/show_bug.cgi?id=1408010 (for Firefox 58),
 Mozilla fixed things so that the protocol handler code would prompt if a
 protocol handler application was configured and then later removed from
 the user's system. However, it seems that our patch for #22618 is
 incompatible with Mozilla's fixed; with Tor Browser in this scenario,
 opening the protocol handler silently fails and a JS exception is logged
 to the browser console:
   NS_ERROR_FILE_NOT_FOUND: ... external-app-blocker.js:129
 (external-app-blocker.js is part of Torbutton).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/28375>
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