There are at least four ways to get a bunch of tabs at startup: (1) Restoring the previous session Were these the tabs open when you quit Firefox? They might be restored automatically if you have that set on the Preferences/Settings page, or if Firefox crashed during shutdown and it's doing a crash recovery. (2) Multiple home pages Firefox allows you to set your home page to multiple URLs, and each will open in its own tab. To clear your home page setting, you can go to the Preferences/Settings page, Home panel, and clear out the box below Custom URLs. More info on setting the home page: [[How to set the home page]]. (3) Pinned tabs Pinned tabs are restored as long as their window was open when you quit Firefox. More info on Pinned tabs: [[Pinned Tabs - keep favorite websites open and just a click away]]. (4) Modified Firefox shortcut with URLs added I don't know if this is a thing on MacOS; it is a thing on Windows. ---- If it's not one of those, maybe one of your add-ons is doing it??