From my experience and Mediafire staff concur with this (from a recent interview by the CEO), is that everything hosted on MF lasts forever UNLESS it is reported to be removed. Manga, is an extremely rare case anyway. It's just mostly anime, films, music, and sometimes software. I have quite a lot of manga archived on my MF account, including the Okusan raws that I once shared (posted by someone else on jcafe) via my blog. At one point I even had 23 volumes of Countach without a single download by anyone in 2 years (deleted it myself though). That said, I'd stick with MF because it doesn't affect the manga scene in comparison to the anime, film, and music.
If MF is not going to used for what ever the reasoning is, the only safest option I see are cloud storage providers such as Dropbox, SkyDrive, Box.net and many more. Others are either unreliable or pay hosts with time limits and non-parallel downloading web-sites.