Websites that aren't in my history, cache, cookies, or bookmarks are being autocompleted and I don't know how to remove them.
The old shift+delete thing doesn't work.
- User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:66.0) Gecko/20100101 Firefox/66.0
On Windows Delete alone should be sufficient. Do these items have a star next to it, indicating it is a bookmark?
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 then 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
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:preferences page.
- Help -> Troubleshooting Information -> Profile Directory:
Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
If you use Sync then best is to disconnect Sync temporarily during troubleshooting.