Here's how we can find the pref name for this option:
First, we type the string into Searchfox (https://searchfox.org), which is a code search tool for Firefox. Here are the results: https://searchfox.org/mozilla-central/search?q=backlogged+crash+reports&case=false®exp=false&path=
In this case, we're looking for a
.ftl file, with the string and string ID. We use these files for localization, so that, for example, about:preferences can refer to the string ID for that option, without including the string directly in the HTML or XUL.
For this bug, we have a result in
preferences.ftl, which is a Fluent file containing the pref strings. Fluent is our newest l10n system, and documented here, if you're interested: https://searchfox.org/mozilla-central/source/browser/locales/en-US/browser/preferences/preferences.ftl
So we can see that the string ID for that option is "collection-backlogged-crash-reports" (https://searchfox.org/mozilla-central/rev/465dbfe030dfec7756b9b523029e90d48dd5ecce/browser/locales/en-US/browser/preferences/preferences.ftl#955-957). Let's search for that: https://searchfox.org/mozilla-central/search?q=collection-backlogged-crash-reports&case=false®exp=false&path=
This time, we get a result in
privacy.xul, which is the Privacy screen in about:preferences. Clicking that result, we see the checkbox for that option, with a
preference="browser.crashReports.unsubmittedCheck.autoSubmit2" attribute (https://searchfox.org/mozilla-central/rev/465dbfe030dfec7756b9b523029e90d48dd5ecce/browser/components/preferences/in-content/privacy.xul#690-694).
So now we know that the about:config pref name for "Allow Firefox to send backlogged crash reports on your behalf" is
And all we need to do is add another default pref for Sync: