"How can I fix it?"

Unfortunately, it seems like either your Firefox install got corrupted, there's a external application preventing the file to be accessed (usually an anti-virus) or even an actual harddisk failure or filesystem problem (less likely).

Going through the first guess (the other ones are harder to diagnose and provide remote assistance), I recommend reinstalling Firefox. This seems related with the application itself so reinstalling will not remove any of your user data (bookmarks, extras, etc.).

If that doesn't work:

1. check your security software - mark Firefox as a trusted application if needed
2. go to that directory (C:\Program Files\Mozilla Firefox\browser) and check if the file omni.ja exists; if so, try to copy it to some other location
3. post back the results

Good luck!

The browserconfig.properties file was used in older Firefox versions to set a localized homepage. This has been changed in Firefox 63 to have support for non-localized homepages, so seeing a reference to this file is likely a leftover from an older Firefox version.

You can check the browser.startup.homepage pref on the about:config page and reset this pref to the default value.

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox