The best way to do it would be to uninstall Mozilla browser and again reinstall it. However, if you don't want to do that there is a way to solve this problem. Just reset Firefox. It will delete all your personal data and settings from Firefox, but leaving the most important data like bookmarks, browsing history, passwords, cookies, web-form auto fill information and personal dictionary.

To make it possible, click on Firefox and then Help and then select Troubleshooting Information.

Now click reset Firefox in the upper right corner of the Troubleshooting Information page.

Sometimes, you may feel that the pages are not loading properly in the beginning. However, gradually the problem will be solved. In future if you face the same kind of problem, just reset it and to solve the problem of the pages loading slowly, you need to use some advanced settings.

You need to change the basic browser configuration for that. Although Firefox is made to give you the maximum speed possible, changing some important settings will work even better. Read How to Make Browsing Faster Using Mozilla Firefox?

Uninstalling Firefox, without deleting all user "customizations and data", isn't gonna do anything to help the user with what he asked help with! Plus he will lose his bookmarks, passwords, and other personal data by using that option in the un-installation routine.

And that article How to Make Browsing Faster Using Mozilla Firefox is filled with some of the same FUD as similar articles floating around the internet for like 10 years.

There really is no "simple way". I have never used most of the extensions which you have installed, so I have no idea in what format the data for every extension is stored. If you could mention exactly which extensions you specifically want delete data from, one of us should be able to guide you by steps for each extension. Some data might be stored in a sqlite file in the Profile folder, with a "name" that will identify the extension, while others may use the prefs.js file which is the "real old" way of doing things, and a some may even store data elsewhere on the drive (Clippings is one that I use which stores its data in C:\Documents and Settings\User\ so that data is available for other Mozilla programs like Thunderbird or SeaMonkey).

So the easiest thing to do might be to create a new Profile, install those extensions all over again, and "migrate" your personal data fron the old Profile to the new one.

1. Use the Profile Manager to create and remove Firefox profiles
Do not remove the current Profile before migrating your personal data to the new Profile!

2. Recovering important data from an old profile