The trouble with Firefox (and Internet Explorer for that matter) is that it's messy. Computer programs haven't "grown up" yet and that shows in the way they store things. I often say that they make a teenager's bedroom look and https://digitalsoftmedia.co.uk that most definitely applies with your browsing history.
Firefox offers a menu option (which can be invoked with a key combination) to delete your history.
Go to the Tools menu and the "Clear Recent History" option.
Or you can press the Ctrl, Shift and Delete all at the same time if you prefer doing contortion.
This will bring a dialog box that has lot of innocent looking things to tick and select.
It's epochal to remember that all the actions with clearing internet history can't easily undone - the only reliable-ish way I've found is to expend Windows System Restore - so you press the "Clear Now" you'll have gone past the of no return.
The pinnacle option is the most innocent - you can select the range to clear. This doesn't cerebrate you much in the way of options (Google Chrome is a mete out better in this respect):
You can choose the default value of Everything - this will do as you expect and scrub your browsing history since you've installed Firefox.
If deleting the of your browsing history is much to bear, Firefox offers the last hour (which should get of that incriminating porn viewing session!), two hours, four hours or today.
If you don't like this pretty much "all or nothing" act then do the browsing you'd want to remove with Google Chrome which offers the past hour, day, or 4 weeks as its options.
Next up, it's time to choose the things you want to clear. I told you this was messy!
Firefox will pre-tick "Browsing