I had a Librewolf appimage that the AppImageLauncher integrated with the menu. Later on, I installed Librewolf from the AUR, and deleted the Lw appimage via AppImageLauncher. However, I now have a menu entry for Lw installed via the AUR, as well as an entry for the non-existent appimage.
How can I delete the latter? I looked in the Cinnamon system settings but did not find anything for that.
look in /usr/share/applications or /home/your_username/.local/share/applications, in this folders you will find the menu entries.
Edit:
if you are not sure which menu entry you should delete, open it with an editor and post us the contents.
In /home/my_username/.local/share/applications there is nothing of the sort.
In /usr/share/applications there are the various icons, incl. Librewolf. The latter is the AUR version I think; the Lw appimage icon (actually there are 2 suck icons) are not in that folder, but they do appear in the main menu.
Also, in Cinnamon, you could right-click on the menu icon on the panel >> Configure >> Menu >> Open menu editor >> Internet
Then look under “properties” of each item to see which one is which.
I think AppImageLauncher will create the .desktop file only for the user and not system-wide.
Good one. While looking through /usr/share/applications I came across a file Cinnamon Menu Editor, which is the same 
Unfortunately, there is only 1 Lw entry in there.
So, how can I delete it from the menu? I have tried right-clicking on the items in the menu, but there is no otion to Delete.
I would have thought it should be under:
/home/my_username/.local/share/applications
as mentioned by @pycrk as well.
Install fd from the repos:
fd -i librewolf / | grep -i desktop
Or mLocate is another option 
I installed fd, then ran the command, and got this output:
/usr/share/applications/librewolf.desktop
/var/tmp/pamac-build-peter/librewolf/librewolf.desktop
The 1st line is the AUR version, I think, and the 2nd line is the appimage. I just deleted the librewolf.desktop file in the 2nd one, but the menu item remains. Does that mean it rquires a reboot?
That is very odd - I was expecting two entries - try a reboot, but don’t think it will help.
Did you choose any location for storing the appimage files when you first ran AppImageLauncher?
If you didn’t, the default location is /home/$USER/Applications.
Look in there to see if you will find any remnants of files related to Librewolf Appimage.
The location I chose at the time was Applications in the home folder. Unfortunatly there is nothing left - I deleted the appimage using AppImage Launcher, so it was done properly.
I have deleted the one librewolf.desktop file, I am going to reboot.
@xircon you were right, reboot did not delete the menu entries.
I did get a notification that Cinnamon could not … (cannot remember the term) because several applets or desktlets are missing. Mind you, I have not noticed anything different to Cinnamon, so far.
look where appimage is mounted and look there in the folder for a .Desktop File.
Do you have a directory ~/Applications? If so what is in it?
The appimage is not mounted anymore, it’s gone, deleted.
And .trash is empty.
What is Bastyon?
Open the appimage File with a Editor, i think that is the File you should delete it.