Update user.js

✅ reEnabled download and share images
✅ Enabled HSTS preload list
⛔️ Disabled Telemetry Coverage (hidden pref, I'm not sure that it is also present in fennec but for safety I added it)
⛔️ Set max popups from a single non-click event from 2 to 0 (default 20) 
⛔️ Removed limits of the amount of entries in your DNS cache
⛔️ Disabled navigator.registerProtocolHandler (can no longer be used on insecure sites)

 (thanks to https://github.com/v1nc for these suggestions)

user.js

@@ -40,7 +40,7 @@ user_pref("dom.ipc.plugins.flash.subprocess.crashreporter.enabled", false);
 // Pref : Disable sending the URL of the website where a plugin crashed
 user_pref("dom.ipc.plugins.reportCrashURL", false);
 // -------------------------------------
-// Pref : Disable telemetry
+// Pref : Disable Telemetry
 user_pref("toolkit.telemetry.enabled", false);
 user_pref("toolkit.telemetry.debugSlowSql", false);
 user_pref("toolkit.telemetry.reportingpolicy.firstRun", false);
@@ -48,6 +48,11 @@ user_pref("toolkit.telemetry.server", "data:,");
 user_pref("toolkit.telemetry.server_owner", "");
 user_pref("toolkit.telemetry.unified", false);
 // -------------------------------------
+// Pref : Disable Telemetry Coverage
+// https://blog.mozilla.org/data/2018/08/20/effectively-measuring-search-in-firefox/
+user_pref("toolkit.telemetry.coverage.opt-out", true); // [HIDDEN PREF]
+user_pref("toolkit.coverage.opt-out", true); // [HIDDEN PREF]
+// -------------------------------------
 // Pref : Disable collection/sending of the health report (healthreport.sqlite*)
 user_pref("datareporting.policy.currentPolicyVersion", 0);
 user_pref("datareporting.policy.dataSubmissionEnabled", false);
@@ -122,7 +127,6 @@ user_pref("browser.chromeURL", "");
 user_pref("general.useragent.updates.url", "");
 // -------------------------------------
 // Pref : Block unwanted connections
-user_pref("extensions.getLocales.get.url", "");
 user_pref("identity.sync.tokenserver.uri", "");
 user_pref("media.decoder-doctor.new-issue-endpoint", "");
 user_pref("network.trr.confirmationNS", "");
@@ -152,14 +156,11 @@ user_pref("extensions.systemAddon.update.url", "");
 // Pref :
 user_pref("devtools.devices.url", "");
 // -------------------------------------
-// Pref : Maximum pop launch at the same time
-user_pref("dom.popup_maximum", 4);
-// -------------------------------------
 // Pref :
 user_pref("layout.accessiblecaret.hapticfeedback", false); // [DEFAULT: true]
 // -------------------------------------
 // Pref :
-user_pref("dom.registerProtocolHandler.insecure.enabled", true);
+user_pref("dom.registerProtocolHandler.insecure.enabled", false);
 // -------------------------------------
 // Pref : Updates addons automatically
 // https://blog.mozilla.org/addons/how-to-turn-off-add-on-updates/
@@ -293,7 +294,8 @@ user_pref("webchannel.allowObject.urlWhitelist", "");
 user_pref("browser.download.folderList", 2);
 // -------------------------------------
 // Pref : Enforce user interaction for security by always asking the user where to download
-user_pref("browser.download.useDownloadDir", false);
+// [FENNEC] Fix for images not downloading
+user_pref("browser.download.useDownloadDir", true);
 // -------------------------------------
 // Pref : Disable adding downloads to the system's "recent documents" list
 user_pref("browser.download.manager.addToRecentDocs", false);
@@ -520,11 +522,11 @@ user_pref("browser.formfill.enable", false);
 // Pref : Blocking GD Parking Scam Site 
 user_pref("network.dns.localDomains", "librefox.com");
 // -------------------------------------
-// Pref : Disable HSTS preload list (pre-set HSTS sites list provided by Mozilla)
+// Pref : Enable HSTS preload list (pre-set HSTS sites list provided by Mozilla)
 // https://blog.mozilla.org/security/2012/11/01/preloading-hsts/
 // https://wiki.mozilla.org/Privacy/Features/HSTS_Preload_List
 // https://en.wikipedia.org/wiki/HTTP_Strict_Transport_Security
-user_pref("network.stricttransportsecurity.preloadlist", false);
+user_pref("network.stricttransportsecurity.preloadlist", true);
 // -------------------------------------
 // Pref : Disable insecure TLS version fallback
 // https://bugzilla.mozilla.org/show_bug.cgi?id=1084025
@@ -534,9 +536,6 @@ user_pref("security.tls.version.fallback-limit", 3);
 // Pref : Only allow TLS 1.[0-3]
 // http://kb.mozillazine.org/Security.tls.version.*
 user_pref("security.tls.version.min", 2);
-// -------------------------------------
-// Pref : Limits the amount of entries in your DNS cache which can give someone who has access to your computer a list of websites you visited
-user_pref("network.dnsCacheEntries", 100);
 //
 // >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
 // Section : Block Implicit Outbound
@@ -876,7 +875,9 @@ user_pref("browser.link.open_newwindow.restriction", 0); // [DEFAULT: 0]
 user_pref("dom.disable_open_during_load", true); // [DEFAULT: true]
 // -------------------------------------
 // Pref : Set max popups from a single non-click event
-user_pref("dom.popup_maximum", 2); // [DEFAULT: 20]
+// [NOTE] Non-click events should never spawn a popup?
+// http://kb.mozillazine.org/Dom.popup_maximum
+user_pref("dom.popup_maximum", 0); // [DEFAULT: 20]
 // -------------------------------------
 // Pref : Limit events that can cause a popup
 // http://kb.mozillazine.org/Dom.popup_allowed_events