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.
When the Firefox snap package has been updated a notification will appear that Firefox was updated and is ready to launch.
Another noteworthy feature of pending updates for Firefox is that they can be installed automatically without user interaction. For example, I have closed Firefox, locked the screen, and left the computer for a few minutes, and then when I came back I received a notification that a pending update to Firefox had been installed in my absence.
The Firefox update notification has changed since this question was asked. Now it looks like this.
The bottom notification says that Software updates are available to be downloaded. When I ignore this notification on my Ubuntu 22.04 the software updates are downloaded automatically. The top and more recent notification says:
Update available for Firefox.
Close the application to update now. It will update.
As soon as I close the Firefox window, Firefox starts to update, and a few seconds later I get another notification that says:
Firefox was updated.
Ready to launch.
The second screenshot in this answer shows an image of this notification which is the third of three Firefox update notifications. I consider these 3-step notifications to be an improvement on the original notification because because when I close Firefox it only takes a few seconds for Firefox to be updated and ready to launch. If I select History -> Restore Previous Session in Firefox, then all my tabs from the previous session come back.
Best Answer
It's a bug (Firefox Broken Copy Link Key) and should be reported to Mozilla. It's not something that can be fixed by - or is a fault with - Ubuntu. The same bug exists with both snap and APT versions.
There's a link here where this can be simply achieved.