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

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#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                    |     Resolution:
 Keywords:  ff60-esr                  |  Actual Points:
Parent ID:                            |         Points:
 Reviewer:                            |        Sponsor:
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Description changed by mcs:

Old description:

> 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).

New description:

 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 fix; 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#comment:1>
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