I have verified that Telerik_UI_for_ASP.NET_AJAX_2019_1_211_Dev_hotfix.zip  does fix this issue in my situation.  I've posted a before and after screenshot. Thanks for your hard work resolving this.

-Mark

I've put together a sample project that allows you to reproduce the error.  Note this is not the only way that it can be reproduced, but i suspect that if you fix this you will fix it everywhere.  This example demonstrates the issue with check boxes and with radio buttons.  You will need to add the Telerik.Web.UI and Telerik.Web.UI.Skins dll's to run this project.

Steps used to create this project.
1. Create a new ASP.Net WebForms App in VS.Net with all default settings
2. Add references for Telerik Web UI and Telerik Web UI Skins Assemblies
3. Add minimal references to web.config for Telerik to work. 
4.  Add the Telerik Form Decorator to the Site.Master

        <telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" RenderMode="Classic" />

5. Modify the default.aspx page to add 2 repeaters, one displaying a list of asp:checkboxes  and one displaying a list of asp:RadioButtons.
6. Run the project in Firefox Version 63 or higher (I've verified that the issue still exists in version 64, but it does not happen in versions < 63).

Output screenshot is attached.  The default website created by VS.Net is quite bloated so i can't upload it here (file > 10MB). But i wanted to include it to show how easy this is to replicated. It's can be replicated in a default website  with minimal work.  These changes can be made in < 5 minutes.  So instead of attaching the project here, I've uploaded it to an Azure file share:  

https://mdk8804.blob.core.windows.net/uploads/CheckBoxTest.zip 

HTH
-Mark

So not trying to be too Captain Obvious Rumen, but the ticket clearly says the issue is in FireFox v63+.  As you have noted there is no issue in Chrome.  

This is very easy to replicate:

https://demoqa.eadoptonline.com/checkboxtest.aspx

I'm having the problem even with RenderMode="Lightweight", with this RenderMode, the problem also occurs in Chrome.

deleted by the.milak

Hi,
   If add styles which mention above, in IE all controls not appear,
IE screenshot  - http://prntscr.com/lh7p91
Fire Fox - duplicate controls removed but i got square boxes with letters screenshot -http://prntscr.com/lh7pyv

this is style code 
    <style>
           .rfdCheckboxUnchecked, .rfdCheckboxChecked {
               padding: 0 !important;
               background-image: none !important;
           }

.rfdRadioUnchecked, .rfdRadioChecked {
padding: 0 !important;
background-image: none !important;
}
       </style>

Suggestion by Roman BAUER using @-moz-document url-prefix() does not work for checkboxes in a RadComboBox.  It causes the text and checkbox to overlap. See the attached file.

#4 - the style to hide the background image, causes the checkboxes to disappear completely in Internet Explorer.

this also works:
@-moz-document url-prefix() {
            .rfdRadio input[type="radio"], .rfdCheckbox input[type="checkbox"] {
	            position: fixed !important;
            }

        }

For radio button you can use
.rfdRadioUnchecked, .rfdRadioChecked {
padding: 0 !important;
background-image: none !important;
}

Thanks for looking into this.  Please note that this problem also exists with Radio Buttons as well as check boxes.  

-Mark