In Firefox, you can set an option to clear history. To revise or remove this setting, try here:

orange Firefox button or classic Tools menu > Options > Privacy

Toward the bottom of the History section: uncheck "Clear history when Firefox closes" if it is checked.

In addition to that feature, history could be cleared by:

• An add-on
• External software
• Other humans

Tracking down which ones are involved may take some serious sleuthing.

how do I stop other people from deleting web history

how do I stop other people from deleting web history

You could create separate Windows logons so that Windows enforces separate Firefox settings and history. Otherwise, if you let people share your Firefox settings and history (i.e., your Firefox profile) you need their cooperation. Otherwise they can do anything you can do.

Block and unblock websites with parental controls on Firefox is a page that might possibly help :) There isn't any way to keep someone from deleting history in Firefox (nor IE or Chrome, or any other browser for that matter). But if you want to keep tabs on where someone is visiting there are options.