If a firefox widget had anything to do with the disappearance of your desktop, try deleting ~/.mozilla/firefox. That directory will be rebuilt when you reboot.
When you boot and the grub screen appears, press e to edit the boot command grub uses. When the prompt appears, type in "init 3" (without quotes) and press enter. Then will boot you into command line mode.
The directory to delete is in a hidden directory .mozilla in your home directory. The tilde in the path given above is your home directory. You can use the tilde or type in /home/<your-user-name>: they mean the same thing.
From the opening cursor when you log in to init 3, enter "cd .mozilla" (without quotes). Then enter the command ls. That will list files/directories. You should see a directory named firefox (all lower case letters). You should copy your firefox bookmarks.html to another location for safekeeping. In the .mozilla/firefox directory, there is another named <some random string>.default. Your bookmarks are inside that directory.
cp ~/.mozilla/firefox/<some random string>.default/bookmarks.html ~/.mozilla
will save your bookmarks to ~/.mozilla. You copy them back to firefox later. Just replace <some random string> with the actual string of characters you find there.
Then enter the command "cd ~/.mozilla" to move out of the firefox directory. Enter the command "rm -rf firefox" to delete the firefox directory and it's contents. If it's an offending widget that's causing the problem, that should delete it.
Then enter the command init 5 (or startx) to start the graphics mode. If a firefox widget was causing the problem, your desktop should now be restored.