Usually a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.
(2) user.js file
This is a configuration file that overrides changes you made in Firefox every time you exit and restart the browser. To track down and remove such a file, check out the steps in this article: How to fix preferences that won't save.
Note: if it comes back, the home page URL might provide a cue as to what program on your computer is creating/editing the user.js file.