Problem when launching Firefox

You did not install Firefox properly. You need to drag it to your Applications folder, as the diagram shows in the disk image. If you have a link to Firefox in your Dock, remove it (by dragging it off, so it goes poof) and replacing it with the one you copied to your Applications folder (while Firefox isn't running).

 

Have a look at this guide: Installing Software.

 

drag the firefox installer in the trash can. It's that thing beneath your macintosh HD.

 

Did you install it correctly?

1) Download the .dmg file which contains Firefox
2) When the .dmg file opens you see that window in your screen shot
3) Click and drag the Firefox icon to the folder icon to the right
4) Eject the .dmg (= the image file) by either right clicking it on the desktop and choose eject or go into Finder and eject it from the side bar to the left.
4) The Firefox.app (the whole program) can now be located in the Applications folder. It's usually a shortcut to the folder in side bar to the left below the Places category.
5) To create a shortcut to Firefox in your dock drag it from the Applications folder to the dock.