Try this. https://support.mozilla.org/en-US/kb/how-clear-firefox-cache#search

Are you running Firefox in private browsing mode?

Note that in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History) all session data it kept in memory and purged automatically when you close Firefox (all PB mode windows) and you won't be able to clear history from regular mode. So make sure that you do not confuse PB mode history and regular mode history.

If you're unable to delete your browsing history, there could be several potential reasons for this issue. Here are a few possibilities to consider:

Browser settings: Make sure you're accessing the correct settings within your browser to clear your browsing history. Different browsers have different methods for clearing history, so it's important to follow the appropriate steps for your specific browser. Typically, you can access the history-clearing options from the browser's settings or preferences menu.

Permissions: Ensure that you have the necessary permissions to modify or delete browsing history on your device. If you're using a shared computer or a device managed by an administrator, there may be restrictions in place that prevent you from deleting the browsing history.

Software conflicts: It's possible that other software, such as antivirus programs or security software, may be interfering with your ability to delete browsing history. Temporarily disabling such software or adding exceptions for your browser may help resolve the issue.

Browser extensions: Some browser extensions or add-ons can affect the functionality of the browser, including the ability to clear browsing history. Try disabling any extensions you have installed and see if that resolves the problem.

Cache or temporary files: Clearing the browser cache and temporary files can sometimes resolve issues with deleting browsing history. Look for options in your browser settings to clear cache and temporary files, and give that a try.

Browser updates: Ensure that your browser is up to date with the latest version. Sometimes, outdated versions can have bugs or issues that affect the functionality of certain features, including history deletion.

If none of the above steps resolve the issue, it may be helpful to provide more details about the specific browser you're using, the steps you've taken so far, and any error messages or unusual behaviors you're encountering.