Where is this list? On my (Ubuntu) machine I suspect it's ~/.mozilla/firefox/<profile-string-here>.default/browser-extension-data/jid1-<another-string-here>@jetpack/storage.js. However it definitely does not include a list of every domain I've ever visited.

I have Firefox set to clear everything except for bookmarks and saved passwords each time I close it (which is frequently) and there are definitely sites in that storage.js file from previous sessions, but it is not complete. I wonder if another mechanism purges domains after a time?