Easiest would be to use a separate profile to visit those websites or use the portable Firefox version.

You can install a portable (ESR) Firefox version to access websites that do not work with the current Firefox release.
The portable version comes with its own profile folder and doesn't interfere with the currently installed release version.

• http://portableapps.com/apps/internet/firefox_portable

See:

• http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows
• http://kb.mozillazine.org/Shortcut_to_a_specific_profile
• http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox

You can use the -no-remote command line switch to open another Firefox instance with its own profile and run multiple Firefox instances simultaneously.

• http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile

• https://developer.mozilla.org/en/Mozilla/Multiple_Firefox_Profiles

The NoScript extension will disable plugins on sites where you do not trust JavaScript to run. It's less useful for simultaneously allowing JavaScript but not allowing plugins.

Firefox's click-to-play feature allows you to get prompted to approve plugins on a site-by-site basis. However, I have not used it, so I don't know how (in)convenient that might be.

More information on the configuration steps in this article: http://www.ghacks.net/2013/08/07/how-to-get-click-to-play-working-in-firefox-23-or-newer/

(There might be an official support article, too, but I didn't find it in a quick search.)

My first reply failed to address extensions. Firefox doesn't have any built-in controls to prevent extensions from running on particular sites.

Thanks again for the help :)