Some preferences in the Privacy & Security section of Firefox settings are not synchronized by default.

1. Warn you when websites try to install add-ons
   data-l10n-id="permissions-addon-install-warning"
   preference="xpinstall.whitelist.required"

2. Top sites
   data-l10n-id="addressbar-locbar-topsites-option"
   preference="browser.urlbar.suggest.topsites"

3. Remember browsing and download history
   data-l10n-id="history-remember-browser-option"
   preference="places.history.enabled"

4. Site Preferences
   data-l10n-id="item-site-preferences"
   preference="privacy.cpd.siteSettings"

5. Offline Website Data
   data-l10n-id="item-offline-apps"
   preference="privacy.cpd.offlineApps"

Add new lines to the section // Preferences to be synced by default in mozilla-central/browser/app/profile/firefox.js

1. pref("services.sync.prefs.sync.xpinstall.whitelist.required", true);
2. pref("services.sync.prefs.sync.browser.urlbar.suggest.topsites", true);
3. pref("services.sync.prefs.sync.places.history.enabled", true);
4. pref("services.sync.prefs.sync.privacy.cpd.siteSettings", true);
5. pref("services.sync.prefs.sync.privacy.cpd.offlineApps", true);

Please review my proposed changes for further file modification in the repo.

As Lina reported in Bug 1605736 #1 could be harmful to sync. So we can exclude it here.

Hi Artem! Thanks for filing this! I think (2) looks good, but I’d like to discuss 3-5 with the team before we sync them.

I’m worried about those because some users like to have different preferences around data storage for their different devices. It could be surprising to disable history or temp storage on your work machine, for example—or set them to clear automatically at the end of the session—and suddenly find your home computer isn’t saving history anymore. Or, you might not want to save history on one computer, but check the box on the other, and suddenly your first computer is saving history again when you told it not to, oops!

I don’t think we ever documented which prefs are okay to sync and which aren’t. Some folks will expect us to sync some prefs but not others, others will expect us to sync a completely different set of prefs, and I think most just won’t think too hard about it (I’m definitely in this last category! 😌)

One thing we’ve discussed in the past is showing a little icon in Preferences for synced prefs. Because all devices have to opt in to syncing the pref before it’s actually synced, if you wanted per-device prefs, you’d not toggle it on just those devices. But we’re a pretty small team, and working hard on a lot of other projects, so I don’t know when or if we’ll get around to building this experience. For better or worse, manually adding services.sync.prefs.sync.* prefs is the best we have.

Which brings us to, what prefs do we sync by default? I feel more comfortable syncing some of the smaller preferences that you’ve added in the other bugs (thank you, by the way! 😊💯), because the consequences aren’t too serious. But history and site preferences could break more sites (and it’s not obvious why it’s broken), or end up saving data when you didn’t expect it.

Hi Lina! Thanks for such an extended explanation of how things are going from your perspective.
I'm definitely on your side and don't want to cause any issues by syncing some sensitive preferences. On the other hand, I see the sync can be improved and it is possible to include at least some of the old and new preferences without breaking user experience :)
Anyway, my point is that a general user is not diving deep into what is synced and how, while the advanced user knows that there is a way to disable sync for preferences when it's necessary.
So let's take only 2nd (Top sites) from this bug :)
I'll send a changeset later.
And thank you for letting me be involved in a small part of Firefox development :)

https://hg.mozilla.org/mozilla-central/rev/0383faee0dd0