Before you read my comment: I don't know anything about firefox and a lot of what I am saying is based on assumptions. I also don't know enough about programming yet to understand a lot of what this means.
Sorry, but I don't think that theirs anything you can do to fix the current firefox, but I think their may be ways for you to delete it and re install it.
The message basically says that their was an interruption while installing it and the file was stored wrong and as a result is unreadable. The reason you can't delete it is because, with all the data mixed up, it no longer reads as anything recognizable to the computer (Just a guess).
Try moving it to a new folder, and then deleting the folder.