In your Firefox browser click on the History from the main menu bar, there you select the tab "Clear Recent History".
If somehow the menu bar is not showing up, you can simply press Alt to view it.
From there you have to define the Time range that you want to clear, say last 1 day, 1 hr and so on, from the drop-down menu, there again you have to select the range or the particular time since when you have made those changes.
To get better results it is better to clear up your entire cache once in a while, so to do that you need to select the "Everything" tab.
There looking next to the "Details", you can choose the down arrow to get those particular items of the history that needs to be cleaned. If you want to clear the entire cache, select all the items and Click on "Clear Now".
It is good to Exit from all the browser windows and then re-open your browser.
Apart from these, also use CCCleaner or any disk cleaner software to keep your system clean. There the temporary internet files and other affected places also gets cleaned. This will also increase the speed of your system.
Else, also check out whether you have really saved the changes that you thought you made. Sometimes, it may so happen, that we forget to save our work properly.
Hope that works.
