If you have received a notification of a pending Firefox update it will appear as a small bell icon next to the clock in the top panel. Clicking on the bell will show a Firefox pending update notification as shown in the below screenshot.

As soon as you see the Pending update of "firefox" snap. Close the app to avoid disruptions (13 days left) notification (which means you have 13 days to upgrade Firefox yourself or else the Firefox snap package will get an unattended upgrade) close Firefox and then reopen it after updating the Firefox snap package is finished. If the pending update to the Firefox snap package has not started yet, it can be initiated by running sudo snap refresh. If you don't close Firefox after Firefox has finished being updated, then you will be unable to open any new tabs until you close Firefox and then reopen it.