Mozilla (the people behind Firefox) are always testing new features and tools. One of the newest they are testing is an interesting tool named Send. With this you can send files up to 1GB in size that will self-destruct and can be done so using any modern browser not just Firefox.
To use Send all you need to do is go to the Send Firefox Web Experiment page and then drag your file onto that page. Any file up to 1GB can be uploaded. Once uploaded you will get a link that you can then share with whomever you like.
The file is encrypted client side so Mozilla are unable to see the content of the file. The file can only be downloaded once and then the link will no longer work; if it is not downloaded within 24 hours it will self-destruct so you can send and forget about it.