You can see that icon changed. https://raw.githubusercontent.com/xmha97/site-thumbnails/master/web.telegram.org.png

That might be the thumbnail cache that you can find in the secondary (local) location that is also used for the disk cache, see the about:profiles and about:cache page.

This might also be problem with the favicons.sqlite file.

You can check for problems with the places.sqlite and favicons.sqlite files in the Firefox profile folder.

• use "Verify Integrity" button on the "Help -> Troubleshooting Information" (about:support) page

If errors are reported with "Verify Integrity", close and restart Firefox and retry.

I find pinned sites icons on this path: %LocalAppData%\Mozilla\Firefox\Profiles\73iptm4b.default-release\thumbnails Now I deleted all files and everything working fine.