When you surf online you load all kinds of web sites, as these sites load browsers like Firefox save images, the look of the site and other information so that the site will load faster when you visit it next time, this is known as cache.
Other than cache Firefox can also save your browsing history, the passwords you use to login into sites and the preferences you selected on the various sites you visit. After a while all of these can grow in size and make Firefox run slow and sluggish. To delete all of this data and restore Firefox to its full speed you can clear Firefox History and the other data that it stored like the previously mentioned history, cookies and so on.
While there are tools that you can use to clear Firefox history the easiest way to clear Firefox history using the built in option. In the Firefox menu click on Tools and then click on Clear Recent History or you can also use the Ctrl+Shift+Del shortcut for faster access to the option. Once you click on Clear Recent History a new window should appear where you can select the time range to clear and which data you want to clear as well. To select which data you want to clear click on Details and the data types like Browsing & Download History, Cookies, Cache and so on will appear. If you just want to clear Firefox History deselect all other data types and just check History. If you haven't already done so select the time range to clear and click on Clear Now. Wait for Firefox to clear the data and that's it, you can now continue using Firefox like before. All the data you selected should be deleted and you should see a performance boost in Firefox.
Clearing Firefox History and the other data it stores is not only a good way to boost Firefox performance but also it can save you quite a bit of disk space.