Could you please give more details about what pull down menu you mean?

Is this with history or bookmarks?

it is not the bookmark men, it is where enter web sites

You mean the location/address bar drop-down list?

Does that list still repopulate when you start browsing?

Make sure that you do not run Firefox in permanent Private Browsing mode (Never remember history).

• https://support.mozilla.org/kb/Private+Browsing

In case you use "Clear history when Firefox closes"

• do not clear the Browsing History

• Tools > Options > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/remove-recent-browsing-search-and-download-history

• Clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, passwords, and other website specific data.
• Clearing Cookies will remove all selected cookies including cookies with an allow exception that you want to keep.

yes it is the location address bar drop down list and it does not repopulate when you start browsing