Hi, since all the icons seem to belong to Ars Technica, try deleting ALL references to it. To do that, open the 3-bar menu > Library > History, then click the Show All History link at the bottom of the list to open the History-library window. Now type Ars Technica into the Search History field in the top-right corner, and in the search results, right-click on it, and select Forget About This Site. (Browsing and download history, cookies, cache, active logins, passwords, saved form data, exceptions for cookies, images and pop-ups for that site will be removed). Give it a minute or so to complete the deletion. More - Delete browsing, search and download history on Firefox.

I'm not familiar with Linux so hope it will work in a similar way to the Windows method above.

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This could be caused by a problem with favicons.sqlite.

If you use Sync then best is to disconnect Sync temporarily during troubleshooting.

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

In case errors are reported with "Verify Integrity" close and restart Firefox and retry.

If "Verify Integrity" cannot repair places.sqlite, rename/remove all places.sqlite and favicons.sqlite files in the Firefox profile folder. Firefox will rebuild places.sqlite and restore the bookmarks from a recent JSON backup in the bookmarkbackups folder.

• keep a backup copy of places.sqlite in case a new places.sqlite database has to be created

See also:

• https://support.mozilla.org/en-US/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file
• https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory/Folder:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/Profiles

Thanks! Removing favicons.sqlite helped, Firefox has now re-created them. Although some of them are now screenshots, as opposed to favicons, but it's okay, and I also suspect they'll change over time?

BTW, verifying integrity didn't show any problems. I should probably clean up old history anyway, as I noticed in about:support it says there's a limited number of pages it can keep. Mine is several years old, as I mostly navigate by typing in the URL bar and suggestions from history are very helpful.