I guess this is a message from Firefox trying to start, or does it seem to be a message from a different program trying to access/scan/modify that file? The search.json.mozlz4 file stores details about your search engines, including built-in and add-on search engines. If you remove/hide that file, I think Firefox will create a new one with default settings, so you could try that. Here's how: In the Windows system search box, type or paste %APPDATA%\Mozilla then click down into the folder mentioned in the error message: Firefox\Profiles\ibp4oc3n.default-release\ Right-click search.json.mozlz4 and then click the rename icon and insert OLD into the name, for example, searchOLD.json.mozlz4. Can Firefox start normally after that?