Investigate external app handler patch for 84

From !81 (comment 2717325).

There were significant changes in https://bugzilla.mozilla.org/show_bug.cgi?id=1565574, and manual testing shows a console error when clicking of URIs of not allowed protocols like mailto: or irc::

NS_ERROR_NOT_IMPLEMENTED: Component returned failure code: 0x80004001 (NS_ERROR_NOT_IMPLEMENTED) [nsIHandlerInfo.hasDefaultHandler] 2 HandlerService.js:482
    fillHandlerInfo jar:file:///home/user/utils/tor/tor-browser_en-US/Browser/omni.ja!/components/HandlerService.js:482

Testing shows the PDF case seems to be handled as intended (blocking dialog displayed).

We should investigate what is causing this error and double-check the patch is working as intended.

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