Some preferences in Firefox Menu - Options - General are not synchronized by default.

1. Warn you when quitting the browser
   data-l10n-id="startup-restore-warn-on-quit"
   preference="browser.sessionstore.warnOnQuit"
   ++pref("services.sync.prefs.sync.browser.sessionstore.warnOnQuit", true);

2. Always check if { -brand-short-name } is your default browser
   data-l10n-id="always-check-default"
   preference="browser.shell.checkDefaultBrowser"
   ++pref("services.sync.prefs.sync.browser.shell.checkDefaultBrowser", true);

3. Enable Container Tabs
   data-l10n-id="browser-containers-enabled"
   preference="privacy.userContext.enabled"
   ++pref("services.sync.prefs.sync.privacy.userContext.enabled", true);

4. Select a container for each new tab
   data-l10n-id="containers-new-tab-check"
   preference="privacy.userContext.newTabContainerOnLeftClick.enabled"
   ++pref("services.sync.prefs.sync.privacy.userContext.newTabContainerOnLeftClick.enabled", true);

5. Use a background service to install updates
   data-l10n-id="update-application-use-service"
   preference="app.update.service.enabled"
   ++pref("services.sync.prefs.sync.app.update.service.enabled", true);

6. Use recommended performance settings
   data-l10n-id="performance-use-recommended-settings-checkbox"
   preference="browser.preferences.defaultPerformanceSettings.enabled"
   ++pref("services.sync.prefs.sync.browser.preferences.defaultPerformanceSettings.enabled", true);

7. Use hardware acceleration when available
   data-l10n-id="performance-allow-hw-accel"
   preference="layers.acceleration.disabled"
   ++pref("services.sync.prefs.sync.layers.acceleration.disabled", true);

8. Use autoscrolling
   data-l10n-id="browsing-use-autoscroll"
   preference="general.autoScroll"
   ++pref("services.sync.prefs.sync.general.autoScroll", true);

9. Use smooth scrolling
   data-l10n-id="browsing-use-smooth-scrolling"
   preference="general.smoothScroll"
   ++pref("services.sync.prefs.sync.general.smoothScroll", true);

10. Show a touch keyboard when necessary
    data-l10n-id="browsing-use-onscreen-keyboard"
    preference="ui.osk.enabled"
    ++pref("services.sync.prefs.sync.ui.osk.enabled", true);

11. Enable picture-in-picture video controls
    data-l10n-id="browsing-picture-in-picture-toggle-enabled"
    preference="media.videocontrols.picture-in-picture.video-toggle.enabled"
    ++pref("services.sync.prefs.sync.media.videocontrols.picture-in-picture.video-toggle.enabled", true);

12. Recommend extensions as you browse
    data-l10n-id="browsing-cfr-recommendations"
    preference="browser.newtabpage.activity-stream.asrouter.userprefs.cfr.addons"
    ++pref("services.sync.prefs.sync.browser.newtabpage.activity-stream.asrouter.userprefs.cfr.addons", true);

13. Recommend features as you browse
    data-l10n-id="browsing-cfr-features"
    preference="browser.newtabpage.activity-stream.asrouter.userprefs.cfr.features"
    ++pref("services.sync.prefs.sync.browser.newtabpage.activity-stream.asrouter.userprefs.cfr.features", true);

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

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

Sure, those sound good! I’m not sure about some of them (1, 5-7 and 10), but I don’t know nearly enough about them to say either way. Let’s just sync all of these for now, and we can always revert later if it’s too surprising; it’s a pretty low risk patch.

I see the possible issue with 2nd - "Always check if { -brand-short-name } is your default browser".
When using a corporate PC with restricted permissions and some other browser set by default, this preference might cause a conflict.
So I think it's better to avoid syncing this option.

I agree with your precautions regarding 6 and 7th. Maybe it's not a good idea to sync hardware related preferences, so I'll exclude them too.

For other options, I'll send a changeset shortly :)

I also skipped #5.

https://hg.mozilla.org/mozilla-central/rev/6bda1802be5b