When you install Firefox, it will automatically search for plugins, such as Flash. You can disable plugins you don't want Firefox to activate, but removing them from your system is a chore and not really necessary.

Firefox does not include any extensions by default, so there's nothing to remove. If you previously had Firefox on your system and didn't remove your personal data when you uninstalled, Firefox should find your old settings folder. To run a "clean" install of Firefox, after installing Firefox again, you can create a new profile.

Here's how:

With Firefox closed, start up in the Profile Manager by pasting the following into the search box on the Windows Start menu and pressing Enter:

firefox.exe -P

Click the Create Profile button and accept the suggested location. (To avoid data loss do not choose the location of any existing folder containing data.) Then Start Firefox using the new profile. (More info on the Profile Manager: Profile Manager - Create, remove or switch Firefox profiles)

Does that work?

Note: I recommend not deleting any profiles without first making a backup of all of them, just in case something were to go wrong during the deletion process.

By the way, Firefox sometimes is incompatible with your graphics card drivers and may experience display glitches or even cause blue screen crashes. To deal with that, disable Firefox's use of hardware acceleration for graphics here:

orange Firefox button (or Tools menu) > Options > Advanced

On the "General" mini-tab, uncheck the box for "Use hardware acceleration when available"

You usually need to restart for that to take effect.