Userchrome Toggle от Joolee

The userchrome.css file lets you customize the Firefox interface deeply. While very powerfull, any changes will require a restart of the browser to take effect.

This extension adds a new button to your toolbar that allows you to toggle custom userchrome.css styling on-the fly. The magic happens by including special CSS selectors in your user styles. Manipulating those selectors allows you to enable/disable up to three of your modifications on-the-fly.

The addon accomplishes this by changing the browsers 'title prefix' attribute. Don't worry! By using invisible UTF-8 characters by default, you won't even notice a thing! Toggle a single style or select up to three styles to toggle in a drop-down menu. Hardcore keyboard users can even use customizable keyboard-shortcuts.

Permissions and Privacy:
The "Allow notifications" permission is needed to display a browser notification when you toggle a style. You can disable these in the settings.
As for privacy, this extension doesn't connect to the internet whatsoever. You can check the source code on the 'Support site' linked on this page.