How to setup Firefox for custom CSS.

Open the browser and go to about:config
Search for toolkit.legacyUserProfileCustomizations.stylesheets and change the value to true.
Open about:support
Click on "Profile Folder" -> "Open Folder"
Create a sub-folder named "chrome"
Change into the new folder
Download and save the files from here (any .css file) into that folder
Restart Firefox
???
Profit!

How to setup the custom start page.

Install the nightTab extension See here
Open the nighTab_backup.json file and switch to raw view. (Saving off github wont allow import).
Copy and paste the raw data into a file in the extension directory. Save as nightTab_backup.json.
Open the nighTab settings page.
Click on data, then click import data and select the nightTab backup.json file you previously saved.