Did you verify that the icon is still there by opening the icon in a Firefox tab?

Did you check in the DOM Inspector that Firefox is using the userChrome.css and finds this rule?

Does other code in the userChrome.css file still work?

Any errors in the Browser Console (Firefox/Tools > Web Developer) about this?

Hi cor-el, thanks for your reply

Icon(s) is/are there, I checked this by copy-pasting url.

How to check whether userChrome.css is used?

Here are the errors from the console: Could not read chrome manifest 'file:///C:/FirefoxPortable/App/firefox/chrome.manifest'. Could not read chrome manifest 'file:///C:/FirefoxPortable/Data/profile/extensions/hunspell-ru@dictionaries.addons.mozilla.org/chrome.manifest'. Could not read chrome manifest 'file:///C:/FirefoxPortable/App/firefox/browser/extensions/%7B972ce4c6-7e08-4474-a285-3208198ce6fd%7D/chrome.manifest'. No chrome package registered for chrome://smartbookmarksbar/skin/global/base.css No chrome package registered for chrome://smartbookmarksbar/skin/global/mouseover-off.css No chrome package registered for chrome://smartbookmarksbar/skin/global/hideFoldersNames-off.css No chrome package registered for chrome://smartbookmarksbar/skin/global/hideNoFaviconNames-off.css No chrome package registered for chrome://smartbookmarksbar/skin/global/spacing-3.css No chrome package registered for chrome://smartbookmarksbar/skin/global/autohideBar-off.css No chrome package registered for chrome://smartbookmarksbar/skin/themes/firefoxDefault.css No chrome package registered for chrome://smartbookmarksbar/skin/themes/firefoxDefault_mouseover-off.css No chrome package registered for chrome://smartbookmarksbar/skin/themes/firefoxDefault_mouseover-off_hideNoFaviconNames-off.css No chrome package registered for chrome://smartbookmarksbar/skin/global/base.css No chrome package registered for chrome://smartbookmarksbar/skin/global/mouseover-off.css No chrome package registered for chrome://smartbookmarksbar/skin/global/hideFoldersNames-off.css No chrome package registered for chrome://smartbookmarksbar/skin/global/hideNoFaviconNames-off.css No chrome package registered for chrome://smartbookmarksbar/skin/global/spacing-3.css No chrome package registered for chrome://smartbookmarksbar/skin/global/autohideBar-off.css No chrome package registered for chrome://smartbookmarksbar/skin/themes/firefoxDefault.css No chrome package registered for chrome://smartbookmarksbar/skin/themes/firefoxDefault_mouseover-off.css No chrome package registered for chrome://smartbookmarksbar/skin/themes/firefoxDefault_mouseover-off_hideNoFaviconNames-off.css JavaScript 1.6's for-each-in loops are deprecated; consider using ES6 for-of instead ScriptSurrogate.js:344:1634 No chrome package registered for chrome://forcecontenttype/chrome/skin/forcecontenttype.css Force Content-Type: observer added S4E Calling updateWindow: {985837b9-7da5-4d18-9543-53a78b83ad56} Key event not available on some keyboard layouts: key="j" modifiers="control alt shift" browser.xul Key event not available on some keyboard layouts: key="j" modifiers="control alt" browser.xul Key event not available on some keyboard layouts: key="p" modifiers="accel,alt" browser.xul Key event not available on some keyboard layouts: key="c" modifiers="accel,alt" browser.xul [ABE WAN] Detected WAN IP 24.16.21.189 JavaScript 1.6's for-each-in loops are deprecated; consider using ES6 for-of instead ScriptSurrogate.js:343:198 Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help http://xhr.spec.whatwg.org/ utils.js:37:0 JavaScript 1.6's for-each-in loops are deprecated; consider using ES6 for-of instead ScriptSurrogate.js:345:1634 not well-formed messages.json:1:1 JavaScript 1.6's for-each-in loops are deprecated; consider using ES6 for-of instead ScriptSurrogate.js:344:209 push1.push.hello.firefox.com : server does not support RFC 5746, see CVE-2009-3555 <unknown> mutating the Prototype of an object will cause your code to run very slowly; instead create the object with the correct initial Prototype value using Object.create sync.js:112:4 1428164415567 Services.HealthReport.HealthReporter WARN No prefs data found. This site makes use of a SHA-1 Certificate; it's recommended you use certificates with signature algorithms that use hash functions stronger than SHA-1.[Learn More] 1055701 JavaScript 1.6's for-each-in loops are deprecated; consider using ES6 for-of instead ScriptSurrogate.js:344:209 "JQMIGRATE: Logging is active" common-min.932386b591b6.js:5:29009 about:blank : Unable to run script because scripts are blocked internally. <unknown> "JQMIGRATE: jQuery.browser is deprecated" common-min.932386b591b6.js:5:29350 console.trace(): common-min.932386b591b6.js:5 migrateWarn() common-min.932386b591b6.js:5 migrateWarnProp/<.get() common-min.932386b591b6.js:5 <anonymous> questions-min.feba4198166a.js:3 x.Callbacks/c() common-min.932386b591b6.js:3 x.Callbacks/p.fireWith() common-min.932386b591b6.js:3 .ready() common-min.932386b591b6.js:3 q() common-min.932386b591b6.js:3

Nothing unusual there that is about the userChrome.css file. I do see some errors with extensions like 'smartbookmarksbar' however that could be involved.

Did you try to disable all extensions to check for possible issues? Note that Safe Mode disables userChrome.css as well.

This add-on did the trick for me: Bookmark Favicon Changer