Urlbar search icons disappear after browser restart

Patch for #30845 (moved) fixes this issue on a fresh profile, but not on an already existing profile which was already restarted at least once. On top of that, there is a quite surprising behaviour:

1. Open Tor Browser ESR68 without patch for #30845 (moved).
2. Restart browser.
3. Check that search icons have disappeared.
4. Change pref from #30845 (moved) and restart.
5. Urlbar is unusable, and browser console shows:

1567091784597	addons.xpi-utils	WARN	Synchronous load of XPI database due to getAddonsByType([theme]) Stack: getAddonsByType@re[/gre/modules/addons/XPIDatabase.jsm:1997:21](/gre/modules/addons/XPIDatabase.jsm:1997:21)
installBuiltinAddon@re[/gre/modules/addons/XPIInstall.jsm:4283:24](/gre/modules/addons/XPIInstall.jsm:4283:24)
_openNetworkSettings@jar:file:///tor-browser_en-US/Browser/browser/omni.ja!/chrome/torlauncher/components/tl-process.js:749:11
_controlTor@jar:file:///tor-browser_en-US/Browser/browser/omni.ja!/chrome/torlauncher/components/tl-process.js:584:14
TorStartAndControlTor@jar:file:///tor-browser_en-US/Browser/browser/omni.ja!/chrome/torlauncher/components/tl-process.js:331:10
observe@jar:file:///tor-browser_en-US/Browser/browser/omni.ja!/chrome/torlauncher/components/tl-process.js:133:14

which might be related, but did not investigate yet.