Bug 1506416 - Enable setting Firefox as default PDF reader on Windows. r=mhowell

Comparing to .webp, we already do two of three things needed.  This
arranges the last thing: registering the file association.

Differential Revision: https://phabricator.services.mozilla.com/D76244

browser/installer/windows/nsis/shared.nsh

@@ -574,6 +574,7 @@ ${EndIf}
 
   WriteRegStr ${RegKey} "$0\Capabilities\FileAssociations" ".htm"   "FirefoxHTML$2"
   WriteRegStr ${RegKey} "$0\Capabilities\FileAssociations" ".html"  "FirefoxHTML$2"
+  WriteRegStr ${RegKey} "$0\Capabilities\FileAssociations" ".pdf"   "FirefoxHTML$2"
   WriteRegStr ${RegKey} "$0\Capabilities\FileAssociations" ".shtml" "FirefoxHTML$2"
   WriteRegStr ${RegKey} "$0\Capabilities\FileAssociations" ".xht"   "FirefoxHTML$2"
   WriteRegStr ${RegKey} "$0\Capabilities\FileAssociations" ".xhtml" "FirefoxHTML$2"