A workaround is to disable the embedded jQuery and use an older version (for example, 1.11.1 which is the previous version the suite used): https://docs.telerik.com/devtools/aspnet-ajax/controls/scriptmanager/disabling-the-embedded-jquery
The virualization demo (https://demos.telerik.com/aspnet-ajax/grid/examples/performance/virtualization/defaultcs.aspx) seems to work fine for me under Firefox as well and if this functionality does not work for your project, I advise that you first compare the project implementation to our demo to find the difference causing the issue. My second advice would be to upgrade to the latest version under which the demos run (2019.1.215 at the time of writing). My third advice would be to open a support ticket and send us a simple runnable reproducible that shows the problem so we can properly investigate with the expected response times.
Radgrid Lazy Loading not working in Firefox browser. It's Showing error in console
This site appears to use a scroll-linked positioning effect. This may not work well with asynchronous panning; see https://developer.mozilla.org/docs/Mozilla/Performance/ScrollLinkedEffects for further details and to join the discussion on related tools and features!
Please resolve it.This indicates that the version your app has is R2 2018 that carries jQuery 3.3.1. To fix this, either follow the article linked above, or upgrade to the R2 2018 SP1 (or later) release.