Previously, if someone was upgrading from Zotero for Firefox to 5.0 but
at some point had installed Zotero Standalone and told it not to share
the data directory, 5.0 would use the data directory from the Zotero
profile directory, and people would end up with an empty database with a
Zotero Quick Start Guide from years ago.
This checks for the database with the most recent mtime and uses that
data directory instead. In the Firefox profile, that can be either the
default 'zotero' subdirectory or a custom data directory. If one of the
Firefox locations is used, Zotero prefs are read from Firefox, because
it means that Zotero for Firefox was being used (because if Zotero
Standalone had been in use it would've needed a custom data dir setting
pointing at the Firefox database).
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