How to delete your Google history on Firefox:
Begin by clicking the menu icon with three lines on the browser window’s upper-right side.
Then click on the Library icon from the drop-down menu.
Now click on the History icon that appears in the drop-down menu inside the Library tab.
Click on the View History sidebar and you will see options ranging from Today to Older Than 6 Months.
Right-click on the option you want and press Delete.
For mobile, tap the three-bar menu icon on the upper right side of the browser, then select Settings.
Tap Search, scroll down, and then clear the Show search history checkbox.